It's a mix of things; Deck, AdSense (for all the trouble they're still a major and these days fairly steady source of revenue), mostly-passive Amazon affiliate, and direct support from the MetaFilter community.

We've looked at other ad rubrics and been disappointed with the specific performance we've seen in a couple test cases, but the volatility of the ad economy means that it's certainly something we'll be reexamining continuously as time goes by.