UT3 has some fun ones although that game seems just about dead now.
I'll bet. Unreal Tournament was incredible for mods back in the day...
I have heard that L4D2 has modded servers so I'll definitely pick that up in 2 weeks when I get my new GPU.
Between black Ops and BFBC2, does anyone know which is the better game for mods? Hell, do these games even allow modded servers?