I planned to build something very similar this about four years back. Registered "Ransomware.com" just for the purpose (and still hang onto the domain, because one day I might come back to it). This was about the same time I started in on what is my current business, so it took a backseat.

Anyway, I think it's a solid idea--but the competition is stiff. SourceForge.net takes donations on behalf of its projects, every Open Source project has a PayPal link, Elance/rent-a-coder eats up the very specific work where only one person/company wants it, SitePoint runs "contests" sponsored by big companies. And, of course, your profit comes from taking a small cut, so you have to do a very high volume--hundreds or thousands of projects per day, if you want reasonable revenues for just a couple of employees. If it's a one-man shop, it's a solid business. By the time you take investment, or hire some additional employees, it could get ugly.