I've been playing the Left 4 Dead demo most of this week. Been having a blast with it, too. But the demo's fairly limited in scope; only two maps, then you're done. Fortunately, PC gamers are nothing if not industrious, and already have come up with a few ways to squeeze a little more variety from the demo.
And they're not even "hacks", really, just workarounds. One lets you play splitscreen co-op on the same PC. Another lets you play as the zombies. And perhaps the best one is from the clip above, showing that the demo has bare-bones support for custom maps.