I love Tac Ops! Was one of the most realistic mods out there (it was sooo easy to die).
I seem to remember there was some behind-the-scenes political / financial shenanigans with Counter Strike and the Game of the Year edition bundle that kind of killed it.
Haha realistic... reminds me that one of my favorite mods back in the day was Action Quake which was more action movie styled and less realistic. Last second sideways jumps in and out of the way were its claim to fame, imo.