A module that'd provide straightforward "shorter links" on a web site based on hashes of URLs within it; for instance, http://us-lot.org/12345678 would redirect to http://us-lot.org/something/hashing/to/12345678/. It'd need to support some sort of "make shorter link" functionality that'd show you a page with the shorter link on it. This need not be limited to one site, but that should probably be configurable.
Ben's shortlinks implements this idea using a CGI script and mod_rewrite.