Dota2, the game that has the most user on steam now available from linux. The page says it will be out for mac and linux clients soon but the beta version can be accessible from steam on linux.
Dota2 Test is available on steam for linux | Better HN
Different strokes for different folks, but the gameplay always seems exciting to me because it's always human teams vs human teams. The number of hero combinations and play style variance keep things interesting, IMO.
