I'm helping with a sponsored online hackathon for liberating tabular data out of PDF files. Where would be the best places to find developers interested in participating?
People from ScraperWiki and OpenKnowledge Foundation sure will like it! They develop and maintain a software called pdftables which extracts tabular data from PDFs. There is also an article on ScraperWiki's blog about research in identifying tabular data in PDFs (since PDFs do not have information about data semantic, only positions, font etc.). To contact people directly you see members of OKFN's GitHub organization, ScraperWiki's GitHub organization and also pdftables contributors.
I don't believe that you will find the "BEST" answer, but here are two suggestions.
Crowdcrafting is an online platfrom that enables people to create and run projects that utilise online assistance in performing tasks that require human cognition. The link I gave you is a project for pdf extraction from this site.
2) For issues like this one, I think that Twitter is one option. You can create a google form with name and e-mail and then tweet that you are looking for vollunteers about this project and they have to fill the form. Be sure that you will use the appropriates hashtags.
You can try http://www.reddit.com/r/opendata too. Some of those particular redditors like things like this. In a previous role I've drummed up support through there too.