Have you seen Source Forge? Would be somewhat easy to get projects added in there for people to work on collaboratively. But I do think it would be a decent idea to have a 'Projects' type of section linking to Source Forge projects (or elsewhere) and people asking for others help. I know when I see cool projects when I have some tuits I like to help. I don't think linking anything to XP's is a good idea. The incentive should be helping and learning, not arbitrary digits only 'useful' on a web site.

Cheers,
KM