Zombie Panic! Source is a mod for HL2 which involves zombies (duh.) There are two modes of play: Survival and Objective. In survival the zombie team gets a set amount of lives (based on number of players) and the goal is to either kill off all of the zombie lives (human) or swipe all of those pesky humans (zombo.) In objective the humans must beat the objectives. There is no surviving in objective mode since the zombies have unlimited lives. Each round somewhere between 1-4 zombies spawn (based on number of players) who are randomly picked players.
Although it does have its issues (some weapons are utterly worthless, etc,) it is a really fun game to play. From my experience, you either love it or hate it. Personally, I prefer this game a lot more over L4D, which I also own. So if you do have HL2 and Steam at least give this game a try. It is download able directly through Steam, you just have to find it in their Mods section on Steam. If you do end up playing it or trying it out, give me a shout. Add me to Steam Friends: minimum@cox.net