Nocturne is the oldest survival server, and features simple gameplay!
Athena is another non-PVP survival server, also featuring a world filled with crazy custom biomes. Towny is used to make towns and nations.
Umbra is a PVP survival server, also featuring mcMMO skills.
The Creative server allows players to play in the creative gamemode. It lets players build absolutely anything.
The Contest server is where building contests take place! Check the home page to know when these are happening.