A modern day marine vs insurgent style mod to Half-Life 2 (requires CS:S or HL2:DM) that is freely available through the Steam Store.  With weapons that vary from sniper/marksman to grenade launcher/RPG's, as well as an assortment of machine guns in between, game play typically falls into a push style game (kind of like king of the hill) or a capture/defend different capture points of the map.  With use of iron sights and scopes, but no crosshairs, opinions of the game seem to fall into "I love this game" or "I'm going to burn down the developers homes"^*.

This one is another that may get some play, but it will have to depend on the general mood of the group.

^{* In part a total exaggeration}