I really like the idea. Where free licenses are concerned, I believe this would help “cross-pollination” among projects and facilitate the progress of open source.

It could also work as a (perhaps curated) reference. Sort of like Rosetta Code, but for more high-level features ­– “how have projects X and Y chosen to implement this feature?”

Besides, it could serve an archival function. A quick search on the web reveals a couple of projects on this vein, but they tend to rely on storing official releases. The way I see your idea, it could be made to work in a way closer to what archive.org does for the web, archiving everything it’s permissible to, with the addition of meta information about the projects, their relationships, etc.

Not a small undertaking, but a very inspiring one!