FREE TO PLAY
This is not technically a Steam game, but I found no PC section to add it too. Anyway, LoL has 15 Million registered users and about 2 million or so that play daily. It was created by members of the original Dota team who make Riot Studios. Now the game boasts 82 champions, dozens of items to created different builds, a mastery + rune system, 3v3, 5v5, and a new Capture and Hold mode coming shortly.
My LoL ID is also MrRazzMatazz, I am lvl 25/30 and have played about 250 total games across IA and Normal modes since I started (1-2) months ago.
This game is amazing, download it and enjoy.