Cached Commons lists dozens of scripts under categories like visualization, Ajax, syntax highlighting, HTML5, Flash, and testing, complete with links to project sources, demos, even documenation. Don’t see a script you want? Just fork the project and your script for everyone to share.
It looks like CSS libraries are on the TODO list, just like I had asked of Google in Episode 0.3.2!
Definitely a great idea. In this age of widespread CDN use, it makes a great deal of sense to use cached sources for such commonly used things. The Google CDN for jQuery is definitely a planned bare minimum in my world. Looks like this will accent that really nicely.