Spore, and its expansions.
I know, they're by no means perfect and don't live up to the absurd pre-release hype at all, but I still enjoy using the game as a creative tool. And in many ways, the best members of the online community more than make up for the faults of the core game via their sheer creativity- the various creature and level editors have been used to produce some seriously impressive stuff over the past couple of years.
Besides, any piece of software that's been used for such diverse purposes as adapting Kafka's Metamorphosis, parodying silent movies, telling a brief Jack the Ripper story and presenting a fictional TV show about alien wildlife can't be entirely worthless.