Comment OpenHatch, and the "Teaching Open Source" Wiki (Score 2, Informative) 151
OpenHatch was mentioned previously, but I'll mention it again for completeness sake. I'm now getting a "500 Internal Server Error from it." (Slashdot effect). Also, there's a list of projects with mentors on the "Teaching Open Source" wiki. Furthermore, as people noted, most open source projects could use some help and you can approach those that interest you.
Finally, touting my own horn, I'd like to note that I'm willing to mentor people with their first steps in my own open source projects. Hack on!