Wow... I can't believe no one has mentioned this one...
Fortress Forever
I think its one of the best mods for Source, it has just been overshadowed by the Orange Box.
Anyway, Fortress Forever is VERY similar to TFC! I like it sooo much better than TF2. Its not cartoony, it has nades, etc. I love it and you will too.
Did I mention its totally free? Seriously, you gotta play this mod and play it now!!