League of Legends
I was recently put onto this game by a friend.
League of Legends is a competitive online game set in an imaginative world. Gamers take the role of a powerful summoner, calling forth and controlling brave champions in battle. League of Legends combines the best elements of session-based games with persistent elements of MMORPG's. Enter the Fields of Justice with your most trusted allies as you fight for your right to rule the world of Valoran. There is only one rule of law among those in the League: Winner takes all.
It's currently in a state of open beta/released. What I mean is, you download the game and you can play with only a select number of champions at first. As you progress and play matches, you gain Influence Points which you can use to purchase new champions, abilities and runes. You can also purchase Riot Points for $, which serve the same purpose as the Influence Points, except you normally have to pay a lot more with IP then RP for the same item.
So far, only one arena is available in a 5v5 format, no ranked matches yet. You can look to get matched by the system on your own with 4 other summoners or you can form a group with friends and then be pitted again another group. The gameplay is intense and fast, with a big emphasis on teamwork and coordination being paramount in order to win. On average, matches last somewhere between 25mins and 1hr.