If you're looking for a simple online browser game that can be quite addictive, I'd suggest you give a shot at Mafia Wars. In case you don't have Facebook, then I propose to you a challenge. I Wanna Be The Guy is an insanely hard platform game you can download to your desktop (it is about 60 MBs). It oughta keep you at it for few hours. If you play it on Medium, you're a wuss.