The description to David McNicol’s URL Cache Plugin raises more questions than it answers:
Given a URL, the url_cache() function will attempt to download the file it represents and return a URL pointing to this locally cached version.
Where did he plan to use it? Does he envision the cache as an archive, or for performance? Why hasn’t it been updated since 2005?
It caught my interest because I’ve long been interested in a solution to link rot in my blog. A real “perma-permalink” would be very useful.