In the Multiplayer menu of the game, you can see many different types of servers. Many of them use special mutators/configurations to provide certain game experience. The following gives some examples. Note that servers come and go so you may not find some of them at certain time.
Some servers specialize in large maps for vehicles fight. Some servers specialize in a certain types of map, e.g. maps with city landscapes, space maps, famous Quake maps, etc.
Some servers focuses on one specific game type. For example, duel servers allow only two players to fight each other; there are also CTF-only servers. Defrag servers provides special maps for only Race-related modes.
Some servers only allow shotguns. Some servers give you grenades. Some servers give you unconventional weapons, i.e. new toys.
Hook+InstGib servers give you hooks and a special vaporizer. Some even provide grenades. These are usually played in CTF.