Most important! Press T to open the console and either type and say hello, or enter commands. Mod info and basic commands below:
Essentials: Use the "/sethome" command to set a location for you to teleport to, and then type "/home" to teleport back to that location. If you later decide you want your teleport location to move, use "/delhome" and repeat the process. These commands are free!
Towny: The towny mod lets you protect plots claimed by your town. Type "/towny" to display the general help file and find commands useful for residents, mayors, modifying your plot, etc. You can type "/town spawn" to teleport back to your town, but this costs money!
mcMMO: Basically an RPG mod! Type "/stats" to see your current skill levels. If you're curious about what a skill does, type "/skillname" to display information about the skill
lockette: Protect your chests! If you place a sign on your chest, it'll automatically protect your chest against destruction and theft. Helpful if you're settting up a store somewhere and need to protect your goods.
ChestShop: This one is a bit complicated. If you want to set up a shop, you need to first place a chest and then place a sign above the chest.
Here's a nice guide with pictures! Remember to look at your sign from the point of view of the customer! B = price a customer buys your product at! S = amount customer gets for selling your product! Many people get confused and reverse the prices, and end up paying more for a product they sell (meaning someone can buy a product from them and sell it back and earn money, eventually taking all the merchant's money)
BOSeconomy: This mod adds a virtual currency into the game. You can earn money by selling resources in Pointzero's Merchant Guild and by selling items to other players. You use money to either buy new items, or found new towns or buy plots within those towns. Check how much money you have with the "/econ" command, and give people money with "/econ pay <username> <amount>"
There are a handful of other minor plugins that have minor effects like letting you fell a tree by destroying the base block of a tree and one that automatically replants saplings, so forests aren't stripped bare by vagrants and miscreants.