In light of a recent discussion and nostalgia of past source mods, wanted to bring up one of my favorites. The Hidden is set up where a team of highly trained PMC are tasked with killing subject 617 "The Hidden". The only problem is that the subject has a superhuman ability of being almost invisible. The subject can also jump long distances, developed concussion grenades, and with his insanity wants to kill the task force with a knife. However, the task force also has a wide array of tools to aid in killing the subject. This game mode doesn't require a lot of people, as only one person per round gets to play as the hidden while everyone else is the task force.
And best of all it is FREE.
If you are interested in getting together a group night were we play this game, leave a comment. Otherwise here is a few useful links.
download link for windows