Rei's MinimapGives you a minimap and lets you set waypoints. Has some other features too, like highlighting slime chunks.
Ender Chest and NEIEnder chest is a special chest that stores its inventory in The END, keeping your items everywhere and nowhere at the same time. Each chest can access the same inventory allowing you to transport items across vast distances instantaneously. It even works between dimensions. Each chest has a code represented by the three wool pads on the top of the chest. Any chests with the same code will be linked.
NEI is a very nice replacement for TMI which also doubles as a recipe book. Check out the linked page for details.
Death Chest and ShelfShelf 1.1
Ever wanted to store your items in a way you can see them? Here you go. This block allows you to add up to 9 items to it and the items you put in are shown in the world.
Death Chest 1.1
When you die, if you have a chest in your inventory, it will be placed where you die, or in the closest available spot. The chest will be filled with as many items as possible. Items stored in the crafting slot are still lost, as usual, and any excess that won't fit in the chest will be dropped.
PlasticCraftPlasticCraft adds a new technological tier to Minecraft: Plastic! The mod adds a variety of new items to the game, such as water bottles, microwaves, rubber trampolines, and even plastic shovels.
PowercraftPowercraft is a very nice automation/factory mod with too many features to list here.
SomniaWhen you sleep, time still passes at an accelerated rate. No more spawning creatures nearby because you're not inside! Sleeping while machines are working will let them get a lot of work done while you nap.
PortalgunSimple enough, adds in a portal gun. Now has see-through portals and improved physics, as well as a grabbing function for the portal gun. Don’t shoot the moon!
BuildcraftJust too much to list here. Pipes, oil, engines, etc. Very nice mod, lets you transport items and set up large machines. Updated to alpha 3.1.3 and now includes lasers, robots, logic gates, redstone pipe wires and more!
Optifine C StandardAdded by request, this mod introduces a LOT of graphics options, support for HD textures, better fog, etc. Lots of stuff here.
Wireless RedstoneThis adds in blocks and items which allow and facilitate the use of redstone at a distance. Using this in conjunction with a portal spawner you can easily set up a quick return portal to your home base for example, or have doors which open on a signal from a handheld remote.
IndustrialCraft2Adds electrical power, machine blocks, material processing and many other things. Latest release even has agriculture and plant cross-breeding.
Equivalent Exchange 2Well they’ve updated a lot of stuff and tacked a 2 onto the name. I haven’t even begun to explore the new items and stuff but I did some testing to ensure it all works reasonably well.
Redpower/RedworldA great set of mods which changes world generation to allow volcanos, adds in a few ores and things like basalt and marble, and adds in a new wiring system as well as bluetricity machines. (Note: By default I have set redworld not to spawn copper or tin ore because IC2 will spawn them. If you allow it to spawn these ores you will have two 'different' copper and tin ore/ingot types. While these types are interchangable due to the forge dictionary, they are still annoying to deal with. If you want them to be spawned open the redpower.cfg file in the redpower folder within .minecraft and change "copper=0" to "copper=1" and "tin=0" to "tin=1"
ThaumcraftAdds some fantastic options for breaking down items into raw magic and using it to set up runes and symbols with various effects. Also custom crafted travel portals!
TreecapitatorTrees fall when any block is broken by an axe type tool. (Now includes all mod-material axes and the chainsaw)
Single Player CommandsAdds SMP type commands to singleplayer.
ForestryAdds in machines which are compatible with Buildcraft and will automatically grow and harvest trees, crops and even mushrooms I believe. Also includes some cross-over power items for buildcraft and IC2 such as the electric engine, and gives some alternate sources of fuel.