We started Tweaky.com to help with this exact problem. We only tackle small projects but we've built a system which makes this work for both customers and developers.

Our project managers break a project down into "tweaks" where each one is well defined and $39. We then curate the developers in the marketplace to ensure that only quality people are in there.

Once the tweaks are paid for the developers pick up the jobs they want to work on and get started.