I've thought about this a lot, and I what may work best would be some sort of wiki. Updateable by all. One clip/TGD would have at least the following properties: name, version, description, preview images (1 or a collection), category assignment (within a category tree to be established), parent (if the item is built upon another library item), creator, and a collection of tags. It would be glad if users could add comments and perhaps a rating (for sorting purposes).
Some people would rather not bother with documenting their work, but I am sure others would be willing to jump in and help out. I also think separation of free and commercial work is important, though there should be room to promote people's work (for example by offering two versions of a clip and including a link to the one that's not free to download).
Some organization seems more than welcome.