Left 4 Dead 2 is good with a small group of non-randos, there are tons of addon levels available. It's a co-op FPS where a 4-player team works through levels full of zombies. If you can't get 4 people together, AI players will fill in any remaining team positions, they're not bad apart from having no interest in carrying certain valuable supplies...
Edit: Whoops looks like a canoe resurrected this thread.
