A recent gaming obsession was PiCTOBiTS, the retro-style puzzle game for the DSi. It is addictive! Then frustrating! Then over. (I was sad when it was over.)
PiCTOBiTS is only available for the DSi, but you can read reviews of other cheap and free games for many systems (including your web browser) in the new biweekly Sawbuck Gamer feature that John put together on The A.V. Club website.