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/