Killing Floor
Spend a good four or five hours on Killing Floor this afternoon. A collaborative zombie shooter that lacks the story structure and slickness of L4D but has the bonus of being set in the UK and being a bit more focussed around instant action.
Slightly strangely there are a lot of gameplay elements drawn from Counterstrike: buying weapons and character classes. It works though and it places a very strong emphasis on teamwork which is great.