This is one that has been a lot of fun, it requires the source engine (so Half-Life 2 or Counter-Strike: Source) although it's fun with the mods associated with Gary's Mod rather than Gary's Mod itself.  The two current favorite mods with Gary's Mod has been Trouble in Terrorist Town and Prop Hunt.  There is no extra download for either mod, but you can get Gary's Mod through the Steam Store.

One or more traitor terrorists in a group of terrorists in Trouble in Terrorist Town, with one or more detectives running around with a group of "innocent" terrorists.  This mod is similar to clue in the respects that you are either trying to figure out who the traitor is (and then slay him), or as the traitor trying to figure out a way to slay everyone else without being discovered and slain.  The one who scores the most points by slaying correctly wins, with lost points for incorrect slaying (innocents slaying innocents, or terrorists slaying terrorists I'm assuming).

Prop Hunt is a game that allows the different players the ability to disguise themselves as different items found around the map.  So see a milk bottle, become the milk bottle, see the crate become the crate.  Survive the hunt by the other side till the time runs out, and you score points.  Alternatively, if you manage to find and shoot the hidden disguised prop/user and you get points, shoot the wrong prop and you lose life.  Lose enough life and you may hunt yourself into oblivion.  While this game may not seem too entertaining in text, there is something to be said for the person chasing trying to gun down the jumping and dodging boot or beer bottle.

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.

Team Fortress 2 is the next version of the classic team fortress that has been available for years.  Missing are the grenades of past versions (except with the Grenadier), but gained are updated graphics and taunts as well as the achievement/weapon system.  With the new system comes the ability to get new weapons and attire the more you play, as well as the ability to "craft" a weapon using parts from other weapons.  If you have more money than time and patience, Steam now allows you to buy new weapons for your characters as well.

While it used to be a retail game, steam has recently released the game for free through the Steam Store.