Here is something I've been thinking about for a while but finally got around to hacking together. Site lets you submit your repositories (already added the most popular ones), view all submitted, and also view the apps that are in each repository.
All you have to do is type in the URL of your repository and click submit to add. All the info is pulled from your XML files.
-Build an boxee app that lets you add repositories from inside boxee (help!)
-Add some filtering
-Build the support page (How to build a proper repository etc.)
-Have a built in Repository on the site, that lets you submit Files and Info so no more worrying if you built your Repository wrong.