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.