http://www.naturalselection2.com/

I was a fan of the Natural Selection mod for Half Life back in the day. But it was too bad that it didn't achieve a whole lot of popularity. I thought it was superior to TFC and CS. I just hope I don't run into more cranky admins that kick you for managing to kill a jetpack marine with a skulk

Personally I wish somebody would do a remake of The Specialists mod, but this is cool too.