You may have heard of or even played Realm of the Mad God on your browser before, but now it's going on to Steam for free! It is a co-op MMO bullet hell 2D shooter where you can basically team up automatically with players nearby and kill monsters and bosses together. There are 13 different classes that you can progressively unlock and play, each with its own unique stats and abilities.
My suggestion? Download it now! It's free, it's fun, and it will consume your soul. With a likely increase of player base, you can expect more content to be added to the game in the near future. Get it on Steam here.
Image source: http://www.tigsource.com/2012/02/18/coming-to-steam-realm-of-the-mad-god/