I've written about open-source programming bounties before, and found a new one yesterday: Bounty County, which looks reasonable enough despite the name.
I didn't see any OS X projects on there yet - it's mostly Gnome stuff - but I'm sure they would list a Mac bounty if you sent it in.
Unlike some proposals I have seen for bounty sites, this doesn't appear to do anything other than list bounties - they don't hold money or evaluate completion. Maybe that's all that it takes, but I'm not sure - I think it'd be more tempting to try for bounties if there were guarantees that my time would be well-spent. Think of the difference between selling things on eBay vs. craigslist - they both work, but craigslist always seems a little sketch.