A script that I could run from a cron job each night to validate all my web pages, and mail me about any problems it finds. "Validation" here could mean HTML and CSS validation, accessibility monitoring, link checking, and so on.
For bonus marks: look for a meta tag indicating the pages' owners, and mail them. If I mirror somebody else's page, I don't want my machine to mail them each night, so I need to be able to provide this script with a list of addresses that it's allowed to mail -- perhaps by pseudonyms that the checker maps to real addresses, to avoid leaking too much information.
The concept of "page owner" could be used by other automatic reporting systems -- for instance, a site where every page has a "send comments" form on it, or one that maintains per-page statistics.