A great modern take on the now classic action-RPG/grid dungeon crawler genre. Mount Grimrock is riddled with hidden secrets and challenges that give a great sense of accomplishment and reward after solving them. Combat on the other hand is simplistic, yet engaging as you must constantly be aware of your surroundings. At 10-15 hours and only 1 dungeon, there isen't much replayability besides multiple difficulties/alternate character builds, but the promised dungeon editor will add many more hours of gamelay in the near future.

Despite being underappreciated and relatively underplayed, Alien Swarm is another "must-have" free game on Steam. Despite a single short campaign (consisting of 7 missions); multiple difficulty tiers, a Left 4 Dead-like AI "Director" setting and unlockable weapons increase the replayability on this already solid tactical top-down shooter.