Bukkit

Allows players to choose certain blocks they don't want to pick up anymore

Gives an infinite amount of random spawns for a player to go to

Makes the xp dropped by mobs configurable

Sets a player's max health based on their xp level

Allows players to disable theirs' or others' chats

Allows players to challenge other players to pvp battles for money

Spawns a firework above the player

Makes it so that when you right click with a hoe, you shoot snowballs