Sorry, I did not catch this thread before Geallor. Gamespy is a bit of a touchy bitch meaning it somtimes shows servers and other times it does not.
I use it ocassionally to join Q-3 servers, but not COD because it is so touch and go. Anyway, most times, you can refresh the rooms and get results. You just may have to do it over and over.
I hope that helps in the future. Oh ya, the best way to find Mob servers for me has been the game itself. It has never failed me yet and may be useful in the event Gamespy is down or is acting up. It also has a fav section...peace