Using bounties for motivating people to improve open source projects may be a good approach, but the problem is that I did not find a bounty website that would be successful.
I know about sites like: bountyup.com, opensourcexperts.com or bountysource.com but they do not look to be so active. Anyway, if you think that one of them is good just put it as an answer.
Please recommend only one site per answer and do it only if you would really use it.
Some open source projects use sites like SourceForge.net: Help Wanted to look for new developers. Now, I know that many projects have paid developers - however I wouldn't class them as commercial developers, so I believe that as long as you phrase it right and make it clear you're an open source project, SourceForge.net won't have a problem if you advertise a paid open source developer position.
Larger Open Source projects that do have bounties tend to advertise the fact on their own pages. I've found the direct approach to be the best approach. If you like/use a particular Open Source product, you should be that much more familiar with it and able to collect on the bounty. So, it's a bit of the reverse approach, rather than finding what bounties are available, see if the projects you are interested in have bounties.
Some examples:
Gnome - http://live.gnome.org/BountiesDiscussion
(I would post more, but I'm new and can't post more than a single hyperlink).
Mozilla, Free Pascal, Apache.org are a few others.
kind of similar to the above (it's on a per project basis)
For Ruby I use http://wiki.github.com/rdp/ruby_bounties/ruby-bounties
for anything else I use http://nextsprocket.com/