When I'm doing a lot of Python or Java development, I want fast access to their documentation. Currently I do this by downloading an archive of the documentation and putting it on my local webserver, but this means using a different set of URIs to get the documentation; this is annoying for me, and breaks links into the documentation from elsewhere.
I would rather be able to download an archive of the documentation with appropriate metadata (a "primer pack") and tell my local proxy server about it, so that going to Sun's Java API pages actually served them from the local copy. The metadata should indicate to the proxy how to map URIs to files inside the archive, and contain the normal HTTP metadata.
An interesting problem to consider is how you instruct the proxy to use the archive in the first place -- extra HTTP headers? Explicit Radio Userland-style links to a standard port on localhost?