Some of my friends have gotten me addicted to a game called Gemcraft Labyrinth. It's a tower defense game where you fight waves of enemies with gems that have different powers. It's extremely addictive. The first several levels are easy, but then it gets extremely hard especially with the experience multipliers on. (I suggest getting the red gems as often as you can, they are amazing).
If you want to check it out, here is the link: