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 :)
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?