Bukkit is an up-and-coming Minecraft Server mod that will completely change how running and modifying a Minecraft server is done - making managing and creating servers easier and providing more flexibility. Learning from the mistakes made by other mods, Bukkit aims to be different and fill the void left by them: built from the ground up we've focused on performance, ease-of-use, extreme customisability and better communication between the Team and, you, our users. The overall design of Bukkit has been inspired by other mods and our experience as Minecraft players just like yourselves, giving us a unique perspective and advantage going into the creation of the Bukkit Project.
Wikipedia describes Bukkit as "Minecraft is a sandbox building indie video game written in Java originally by creator Markus "Notch" Persson and now by his company, Mojang, formed from the proceeds of the game. The game is focused on creativity and building, allowing players to build constructions out of textured cubes in a 3D world. The game is divided into two main modes – free Classic and paid Beta – where Classic is focused entirely on construction with unlimited material supply, while Beta requires players to acquire resources themselves, and contains mobs, player health, and additional features and items. The gameplay is inspired by Dwarf Fortress, Dungeon Keeper, and Infiniminer."