Stack Overflow offers free, vote based ads for open source projects. They seem to refresh them regularly, here's the latest thread for this iteration.
I tried 2 experiments when they opened this time around:
Could I convince enough people to upvote an ad that would help find contributors to Ubuntu?
Could I convince another opensource project to submit an ad?
However I've never seen any of those ads here, they run on the trilogy sites.
While I think this offer is relatively well known in SE circles I am guessing it's not so generally known in parts of the OSS community. Mixing these up with the Area51 ads on Ask Ubuntu might be worth a shot, I can think of a few upstream projects that might be interested if it was more widely known, and as the site grows it might be worth trying.
Response to non-Linuxy OSS projects on the ad network:
Why not? If a windows developer trying Ubuntu in a VM has a question and runs into an ad for an OSS project for windows that needs people then that's a win right? And web OSS is mostly platform agnostic anyways.
We're pretty practical here anyway, we wouldn't care if the Unity ad ran on Stack Overflow in the .net tag right?