My organization just rolled out its new site design (which includes a good chunk of moved pages), and I just upgraded my copy of Danny Goodman's _Dynamic HTML_ to 3rd edition ("updated for Ajax and Web 2.0"). So I quickly realized we should have an AJAX script on our 404 pages. Parse the given URL, apply some heuristics, test a half-dozen candidate addresses, forward user to the best match.
Except that somehow I haven't found any prebuilt libraries for this. It *is* an obvious idea, right? I suppose I'll roll my own if necessary, but a system like this really ought to exist already. And I absolutely agree with Douglas Karr that the grandmasters at Google are blatantly falling down on the job by not catching their 404s more effectively.