Latest versions of RBTTInvasion feature a new kind of game called MonsterHunt. People might remember this game from UT'99

All you need to do is place a ThisIsMonsterHunt actore in your map and the mutator will recognise your map as MonsterHunt instead of Invasion. MonsterHunt comes with nodes you can use in Kismet which lets you spawn monsters on specified places, set their health and/or resize them. Along with the current Kismet possibilities you can do almost ANYTHING to create a cool MonsterHunt map.

Dirty from AdrenalineParty has created a beautiful looking map, that really has a Serious Sam feeling all over it. The map is still in development and has no name yet, check out this video to see his map and MonsterHunt in mind blowing action:
http://www.youtube.com/watch?v=aqOlN3v1LVc&fmt=22