Grand Theft Auto III [Steam store page]
The first 3D third person sandbox game in the series. Unfortunately only a hint of a story, so after a few minutes roaming around and a couple missions it lost my interest. I found Just Cause 2 to be a much more compelling sandbox game

World of Goo [Steam store page]
I'm not madly in love with this game, but it's a decent way to pass the time. The puzzles are usually fairly easy, though sometimes it is hard to execute them because of the way the AI makes goo run around the structure