Bukkit

A chat-based version of workbenches, chests, furnaces, brewing stands and enchantment tables.

Bypass invisibility potions.

Make blocks resistant to water or lava or take their resistance away.

Edit your mob spawner how you like.

Allows you to wipe the "repair-memory" of an item.

Define a set of answers for frequently asked questions and easily send them to users.

A set of improvements for the basic tools (pickaxe, ...)

An alternative to the bukkit safeguarding system.

Just a different world generator.