[tantrix]

Such a thing actually already exists! Take a look at BountySource: https://www.bountysource.com/

As a backer you can put cash bounties on Github issues (other trackers to come) and when a project committer accepts a pull request for that issue, the developer gets paid.

On the flip side, we're about to launch Fundraisers, which is very similar to the Kickstarter model: create a project spec and get it funded. We're building specifically for open source, rather than a generic platform.

We'd love to hear your thoughts! (#bountysource on freenode)