Ideally, you should just wait until Left 4 Dead 2 comes out on November 17th. But if you can't do that -- and I know how tough it is -- then you can use Saul's plug-in/hack to unlock the special infected in the Left 4 Dead 2 demo. This will give you an early taste of the jockey, charger, and spitter (pictured, ewww!). Of course, no guarantees that you won't break everything entirely. Just have a look through the comments section to get a sense of what you're in for.