A bounty on issues would also be interesting... sometimes you have an issue you can't personally fix and isn't critical enough to get a contributors interest. $100 might get their interest though :)
Bug tracking software has played with the idea of bug bounties for many years now. However I challenge you to provide a link to a public bug tracker where these bug bounty features are successfully used.
For further reading, there exists plenty of discussion and analysis surrounding bug bounties for security vulnerabilities. Bounty rewards are typically a token gesture that don't even begin to cover the real costs of the developer/researcher. And for open source projects -- many developers are writing code for fun, as a challenge or as an experiment. Are intrinsically motivated developers going to respond positively to extrinsic motivators?
