Left 4 Dead

Yes, this is the game I played at Valve like 3 weeks ago ans it's so f*cking cool. Great teamwork in it (we were taking out levels Swat style while covering each others' backs).

The levels are freaking huge and it's pretty much a get from point A to point B. Each level is extremely varied and you never go through the same area twice.

Zombies are tricky to play as because they can't be too overpowered and kill players too easily since it's "once you're dead you're dead".

Now, death is handled a bit differently however. Once you reach 0 hp normally, you are considered "Down" and are stuck on the ground, with 300hp this time, can't move and can only fire your pistol. A teammate needs to come over and revive you. In fact, there are lots of moments where you need to be helped by teammates (by holding +use on them).

For example there are areas where you need to jump from a roof into a window to keep moving. If you miss the jump, you can grab onto the ledge, and you need to get a teammate to successfully make the jump, and then pull you over. It's sweet.

Oh, this is pretty much like Zombie Master, but instead of a human player controlling the RTS components, it's run by some overlord AI that judges when, how many, what types of zombies to spawn in based on intensity levels of the player. It takes in your health, hits you get, ammo, etc to judge this... So basically it provides the "perfect" and balanced zombies. Plus, you get to play as 4 sweet zombies which is really tight.