THE HIDDEN

Love the game, it's a free HL2 mod and it is a pretty fast and easy download, really fun game check it out here: http://www.hidden-source.com/
Pretty much you are either hunting the invisible test subject or you are the test subject, and as the test subject you are mostly invisible and can leap around and knife people as they try to kill you its a great game check it out if you haven't

Actually its absolutely free of charge with no need for half life 2, and its a pretty amazing mod I must say. I own my own private server running on the master list so you all could join if you wanted. To install the hidden go to the website I listed above, download the hidden files, take the hidden folder and drag it into the sourcemods folder in path
"C:\Program Files (x86)\Steam\steamapps\sourcemods"
Now shut down your steam, reopen it, and it should be in your library but still unplayable. now up in the top left where the search bar is find where it says ALL GAMES and click it, go down to Tools, from that directory find Source SDK base 2006 and download it, it may take a while, but once its done just run it once, close it, and your hidden game is ready to be played.