This is a D&D 3.5 campaign in the vein of "sandbox" games; that is, you have no preset goal.
I think that sums it up rather well, so I'm going to move onto rules.
1. 28 point buy. Start at level 5.
2. You are given free reign with the SRD, the Complete books, and the Tome of Battle.
3. No Tier 1 classes.
4. You get free +LA adjustment based on your class tier. If you choose not to use this free +LA adjustment on a race, you may instead add it to your ability score. Tier 2's get +1 LA, Tier 3's get +2, so on and so forth.
5. No multiclass penalties.
Now for the obligatory copypasta houserule wall. (We're tight on funding today, so the section captioning is provided by Gollum.)
Stingses
Heavy crossbows (including their repeating forms) have their base damage die increased by one step. As this is a modification to the base item, it stacks with any other effects of that type.
Crossbows can be made to utilize the user's Strength bonus by mechanisms that provide more resistance, and recoil when fired that requires a stronger user in order to utilize properly. This otherwise works just like composite bows, including the +75 (light) or +100 (heavy) gold per + of strength bonus on the 'pull'.
Crossbows, but not bows can ignore a number of points of armor or natural armor bonus equal to the strength bonus of the crossbow or the strength bonus of the wielder, whichever is the lower. Crossbows do not bypass AC that is not armor or natural armor, and if the target has less armor or natural armor than you have penetrating power the remainder has no effect. Yes, this does effectively give you Str and Dex to hit with a crossbow against armored foes.
Composite bows and crossbows can have their pull increased, but not decreased by a skilled bowyer. This requires materials of a total cost of the price difference between the current and the new item + 50 gold, 1 hour of time, and a successful Craft (bowyer) check made at the end of this process. The DC on this skill check is 10 + the new composite bonus of the bow + 2 if the bow or crossbow is an exotic weapon. Failing this check requires the bowyer to restart the hour long process from the beginning but their materials remain intact. Failing by 5 or more ruins the materials but does not harm the bow or crossbow. Yes, the DC is intentionally low. That's because...
Precioussssses
Burst type effects (elemental burst, holy burst, etc) have the burst aspect of their damage doubled. The basic on hit damage remains unchanged.
Magic armor, magic shields, enhancement bonus to natural armor items, and deflection bonus to AC items have the cost of said properties cut in half. Thus armor and shields are 500 * bonus squared and natural armor and deflection items are 1,000 bonus squared. Magic weapons keep the same cost.
Vorpal now triggers on a critical hit, not just on a natural 20. Yes, I am aware that lets a fool with IC: Slashing and a Keen Vorpal slashing weapon behead people 45% of the time or so. It's a fucking +5 enchantment, it needs less fail and more awesome. To dual wield with it you need two such weapons, at a minimum cost of 196,000 gold. Also Vorpal was errataed so Death Ward blocks it, and if some dumbass is running around without Death Ward at these levels they deserve what's coming to them. This also makes Heavy Fort block it. See previous comment.
The Bane weapon enchantment also bypasses all damage reduction by creatures of that type, even if you could not normally bypass that type of damage reduction with that type of attack and even if that type of damage reduction normally cannot be bypassed at all! Now non Artificers might actually use it!
Magic Vestment and Barkskin now gives a bonus of +1 per 2 CLs, rounded down, minimum +1 and maximum +8. Shield of Faith now gives a bonus of +2, +1 per 3 CLs rounded down, max 8. Greater Magic Weapon remains unchanged, but if you prepare any of these spells in a slot 4 levels higher than the spell is for you or cast them spontaneously out of a slot 4 levels higher you can instead cast a Mass version of that type, targeting up to one target (people for Barkskin and Shield of Faith, armor or shields for Magic Vestment, weapons for Greater Magic Weapon) per caster level within 30 feet. The exceptions are Rangers and Barkskin, who can cast a mass Barkskin out of a 4th level spell slot and Artificers, who can cast a Mass Greater Magic Weapon out of a 6th level infusion slot.
A new wondrous item exists in the form of the Sniper's Bowstring. Despite its name, it also works fine with any crossbow, including magical bows and crossbows. Any bow or crossbow fitted with a Sniper's Bowstring automatically counts as composite adjusted to their current Strength score. Its market value is 1,000 gold and can be crafted by anyone with a CL of 3, Craft Wondrous Item, and Bull's Strength.
Featses and Skillses
Anyone who would normally have 2 + Int skills instead has 4 + Int skills. Those that already have 4 or more skill points per level remain unchanged.
Untrained Knowledge skills can reveal information with a DC higher than 10. Of course you're still untrained, so don't expect miracles here.
When using the Craft skill to create a non magical item, determine the time it will take to complete the item using the formula for magic items (1,000 gold/day) instead of the existing formula. If this would result in a time of less than 1 day instead use the following formula: Less than 50 gold = 1 hour, 51-250 gold = 2 hours, every 125 gold or fraction thereof = +1 hour. Yes, this does mean you can make most mundane gear rather quickly. Good for you.
Permanent increases to Intelligence provide retroactive skill points. Temporary increases to Intelligence do not grant additional skills.
The Toughness feat grants 3 + HD HP, and cannot be taken more than once.
Improved Toughness grants 3 + HD HP, requires Toughness and a base Fortitude save of +2, and can still only be taken once.
Improved Two Weapon Fighting and Greater Two Weapon Fighting no longer exist.
Two Weapon Fighting grants you a number of offhand attacks equal to the number of main hand attacks you are entitled to, and does not impose additional to hit penalties for a one handed weapon in the off hand. It also no longer has any prerequisites. Happy birthday.
Two Weapon Defense provides an Insight bonus to AC equal to the number of mainhand attacks you can make derived from BAB.
A third feat exists in the Two Weapon Fighting line called Two Weapon Parry. It requires both of the previous feats and a BAB of 11. With it you can 'hold' a pair of attacks from Two Weapon Fighting, and use those attacks as an Immediate action against an attack made on you. This pair of attacks can either be your primary pair or one of your secondary pairs. If either of your attack rolls are higher than the attack roll of the attack, the attack is parried and does not harm you. If both attack rolls are higher, the attack is parried and if the attack rolls are also sufficient to hit the enemy's AC + 4, you hit them normally with your attacks. If neither are higher, the parry fails and the attack is resolved normally. You must choose to use your stored Parry before learning the result of the attack roll. Parries not used by the start of your next turn are lost.
Improved Trip no longer requires Combat Expertise.
Improved Critical and Keen stack. Happy birthday. In addition Improved Critical applies to a category of weapons chosen from the following list: Bludgeoning, Piercing, or Slashing.
If any of the feats that give +2 to 2 different skills were needed somewhere as a prerequisite, now you don't.
All three of the feats that give +2 to a save still do, except that if your base save bonus of that type is at least +6 you also no longer auto fail saves of that type on a natural 1. You must still actually roll high enough to save normally. If your base save bonus of that type is at least +12 you gain the following additional effect: Fortitude/Will: Mettle. Reflex: Evasion. If you already have these abilities for any reason you instead get a 1/day reroll on a failed save of that type. You must choose to use it after you know you failed the save, but before knowing what the result of doing so is. If the reroll is worse than the original roll, well too bad.
Craft feats no longer grant a discount on the gold cost of an item, and no longer have an XP cost. They still cost the usual amount of time, but are granted automatically to anyone with a sufficient caster level.
Iterative attacks from Base Attack Bonus have a penalty of -5, non stacking. Yes, this does mean a 20th level Warblade swings at +20/+15/+15/+15. This penalty can be further reduced with feats.
Full attacks are a Standard action. You still need Pounce in order to benefit from charge based effects on a full attack.
If you have at least 1 BAB, you can Power Attack with any weapon you are proficient in and are treated as if you have the Power Attack feat without actually taking it. Yes, this does include ranged weapons. Call it a Headshot. Also see feat section.
If you have at least 1 BAB, you can Point Blank Shot with any ranged weapon you are proficient in and are treated as if you have the Point Blank Shot feat without actually taking it. Also see feat section.
If you have at least 4 BAB, you can Precise Shot with any ranged weapon you are proficient in and are treated as if you have the Point Blank Shot feat without actually taking it. Also see feat section.
You can become proficient with any weapon by training with it for (7 days - your BAB, minimum 1 day) and making an Int check. The DC is 5 for Simple weapons, 10 for Martial weapons, 15 for Exotic weapons. You cannot take 10 on this check. If this check fails you can try again from the beginning, but if it fails by 5 or more you cannot reattempt to learn that weapon until your BAB permanently improves.
You can become proficient with any armor by training with it for (14 days - your BAB + the armor bonus of the armor, minimum 1 day) and making an Int check. The DC is 7 + the armor bonus of the armor. You cannot take 10 on this check. If this check fails you can try again from the beginning, but if it fails by 5 or more you cannot reattempt to learn that armor until your BAB permanently improves. If the armor has other negative effects such as arcane spell failure or violates Druidic oath you still suffer those drawbacks.
Classeses
Monks suck a barrel of cocks. But if you really want to play one anyways, gauntlets are considered an unarmed strike and can be enchanted the same as any other weapon.
Monks are proficient with their own bodies (finally! They still fail though). They are also automatically proficient with gauntlets, which are considered a Monk weapon and do the listed damage or their unarmed strike damage, whichever is the greater.
Barbarians gain DR x/- equal to their Con modifier or their Barbarian class level, whichever is lower. This is in addition to the DR x/- gained at level 7, 10, 13, 16, and 19 and any DR x/- the Barbarian has from items.
Barbarians of level 11 or higher also have Mettle, but only while raging.
Monks get full BAB. Not that that will stop them from failing.
Paladin Smite Evil can be used x times per encounter instead of per day and applies to any attacks made for one round, including AoOs. In addition if you Smite a target that is neither Good nor Evil the Smite still has half normal effect. Note the change to Smite also applies to any other Smite abilities with the target lines changed accordingly.
Paladins have good Will saves, and every six levels they can take a bonus feat from the following list: Great Fortitude, Lightning Reflexes, Iron Will. Before you laugh, keep in mind what those feats do now.
Paladins and Rangers have full caster level. Their actual access to spells remains unchanged.
Paladin casting is now Charisma based. Happy birthday.
Ranger's Animal Companion progresses at the same rate as the Druid's.
Warlocks can Eldritch Blast as an attack action as long as it is your turn. Happy birthday.
There was a Hexblade fix around written by the creators. I can't find it, but it's in.
Tome of Battle classes, when using a maneuver that specifies they attack once and deliver the effect of the maneuver can full attack with the maneuver, but it will only be used on the first attack of the sequence or the first pair of attacks for those TWF maneuvers. If a maneuver does already allow you to full attack and gain the benefit on each swing you still do. Yes, this does mean Warblades can use a maneuver, then full attack and use a second maneuver while recovering both.
Swordsages get Adaptive Style as a bonus feat. It only applies to maneuvers learned from being a Swordsage. It otherwise functions exactly the same. Fuck you feat taxes.