Trust the Minecraft community to make a mod of a mod. In this case they're currently building a version of DayZ, the brilliant persistant Arma II zombie mod. The more you think about it, the more ...
I've been playing the MineZ public beta, the DayZ inspired, server-side Minecraft mod that chucks 150 players into a zombie-infested open world and expects them all to survive and play nice together.
However, some Minecraft modifications are good for just about any server out there, and they're certainly worth looking into. They may not suit every player depending on the server's theme, but they ...