Ok, the (somewhat) new
Mann vs Machine Co-Op game is exceedingly fun and terribly addictive.
If you haven't heard about it, it is purely cooperative with up to 6 players on a team. Basically, it turns TF2 into a Tower Defense game except that you are a walking, talking tower. The bots come in randomized waves and bots that die drop cash. Cash that is collected is split evenly amongst all players. Cash can then be spent on upgrades for your class at the supply depot (usually best to do it in between waves, but you can do it mid-battle as well).
All the classes are useful, some moreso than others but every one of them has their place. Scouts have a huge radius to pick up dropped cash (it disappears over time), cash heals themselves, and they respawn instantly when they die. Spy sappers disable entire groups of machines at once. Sniper headshots cause explosions (doesn't have to be a kill shot for an explosion).
You won't get enough cash to upgrade all of your weapons. But you certainly can become very powerful by the final rounds if you spend your cash wisely. My sniper was fully charging up shots in 2 seconds, doing 900 points per headshot, and had a huge headshot explosion radius. Great for taking out large groups of bots (and taking down some of the tougher, giant bots).
Highly recommended.