League of Legend's happens to be one of the better free games out there right now.
In fact, it's widely held to be the best free game you can download and play. It runs on a Free to Play with the Micro-transations for those who wish to customize the characters they play, or unlock those other characters faster.
It's a MOBA game, following the old DOTA style with it's own twist, that being accessibility. Running 5v5 player matches is the dominant game mode, with VS AI and the like there, and quite fun, but not the main draw.
With over 100 Champions to pick and play, you've got tons of choices. With regular new content, a heavy competitive scene with your good old Pro-teams and tournaments, it's up there with Starcraft in popularity.
It's also free, but see how long you can resist paying money, it gives nothing but cosmetics but thats what people buy, as Team Fortress 2 loves to yammer on about.
I'm suck on this crappy makeshift computer I've got. I've broken my primary computer recently, and lack the funds to fix it, so I'm not playing it right now, but if I could....oooh if I could. I'd be running around show you people how Junglers do.
And they do Well man, they do well.