A great refinement would be: on the error page, suggest alternate sites with similar content that are still reachable. Or even: for the exact URL visited, sugg

A great refinement would be: on the error page, suggest alternate sites with similar content that are still reachable. Or even: for the exact URL visited, suggest the one page in the remaining long tail that's most like (by some text/semantic measure) the originally-requested page. (Or even: redirect automatically to that page.)