Realm of the Mad God is a co-op RPG game where you shoot enemies and become more powerful. Pretty simple. I started playing around the end of March and now I'm addicted.
Just be warned that you'll rage quit sometimes. You'll probably still come back after a few hours.
So, you could play it here on the website or on kongregate. I suggest making an account on the website then logging onto it on kongregate.
Here's a random video about it: