Online Games:

MiniClip
MiniClip is a classic site for mobile and PC games as well as most of the throwback games you played as a kid.
Cool Math
Yet again, a classic site that you know best as the website that fooled your highschool firewalls into letting you play Run or Bloons.
Addicting Games
This is the holy grail of games that will not only burn through a class, but also leave you feeling dumber than before you logged on.
Kongregate
Kongregate is a solid site with similar games to MiniClip but also some anime stuff if you're into that.
ESPN Games
Chances are you're already on ESPN.com anyway so why not head over to the games section.