If the retro nature of the above photo doesn't send you screaming from your computer, I've got a little co-op game for you.  Realm of the Mad God is a new MMO co-op bullet hell shooter.  What the heck is that?  Why it's exactly what it sounds like.  It's a new HTML5-based browser game that plays like a shooter and has RPG elements.  I actually did some strait up research and played the game for a spell.  Looks like it has the potential to be a real time waster.  Realm of the Mad God is free to play, but of course there is the option to spend real money on it.  You can complete the game without spending a cent.  All the information you could want is right here.