As above really, been getting into this - can be proper scary
If you've not heard of it, it's a mod for HL2. Two teams, one CT's the other is an invisible human with a knife and 3 nades. The 'Hidden' is a bit like Predator in the way the light shimmers when you move, and that you can jump around, hang on to the ceiling, walls etc. CT's have to kill the Hidden, and the Hidden has to kill the CTs by sneaking up to em and giving em the good news with the blade.
It makes for a really tense game as everyone's looking around, then all of a sudden the bloke next to you explodes in a pile of gore, then everyone goes mental a bit like the jungle scene in Predator.
http://www.hidden-source.com/