So there's a source mod Austin and I have been playing a lot lately. It's super fun, scary at times too. Basically, there are a bunch of S.W.A.T. members out to kill a creature called "The Hidden". The Hidden is supposed to kill all the S.W.A.T. members. Sure, it sounds unfair with 1 vs up to 9, but here's The Hidden's perks. The Hidden is about 90% invisible. All you can see of him is a little distortion in the view, like vapor. He also gets grenades for every 3 enemies there are. On top of all that, he has a skill called the Pig Sticker. It basically causes the enemy to explode. Purdy cool, yo!
Download it here - http://www.hidden-source.com/
All you do is download it, install it, and restart Steam. NOTICE, THOUGH. You MUST already own a source game on Steam to play it.