Hidden: Source is a game of one against everyone else.  You are either the Hidden, a genetically altered mostly invisible knife weilding creature hunting the marines, or the marines trying to hunt down and slay the mostly invisible monster with an assortment of differerent ranged weapons.  The question I guess is, what would you rather be the mostly invisible stabber, or one of the group of hunted shooters?

This one is an exciting if semi-tense game, that requires the source engine (Half-Life 2 or Counter-Strike: Source).  Hidden: Source is also external to Steam, but is freely available to download here.