I guess I'll start with myself. I, personally like Cube. It's a concept engine with simple but powerful in game editing. It is designed to make mapping easy and fun.

You can get it at its main page: http://www.cubeengine.com

The bigger the community the better (we could really use some good programers for the prototype engine Sauerbraten.)

PS: Be sure to check out my Cube maps at Quadropolis: http://cube.snieb.com/
"We can categorically state that we have not released man-eating badgers into the area."

-UK military spokesman Major Mike Shearer