Claustrophobia is an Action/Adventure/Puzzle type mod in which you play a rebel in the Half-Life 2 universe fighting and solving his way through areas to get to safety.
This is an excellent and very long mod for Half Life 2. I spent ages getting lost in some levels. It was really great.
My only gripe (and why it doesn't get a 10/10) is that many of the "keys" are nearly invisible unless you're looking directly at them with your flashlight shining and some of the other things necessary to open doors (such as access cards and their associated readers) looked rather out of place.
For example: in a brick wall, during one part of the game, is a "card reader" which appears to be simply four rectangles with "Combine Metal" textures and a "Combine Forcefield" inside the rectangle. It was a little...off.
Otherwise, though, superb level editing and very engrossing. I loved it. :)