Battlefield: 1942 (FREE)
When it first came out, Battlefield: 1942 helped redefine what the modern military shooter could do in multiplayer. The game offered bigger maps with more players, plus an incredibly wide range of usable vehicles spanning the land, sea, and air. EA has just made the complete game available for free through its Origin service. Even if it's just going to sit on your digital shelf collecting dust, it's worth having.