Are 404 errors a missed marketing opportunity for most web designers, or should they be left as a ‘page not found’ marker and nothing more?

Certainly the vast majority of sites would appear to point to nothing more useful than the Apache or ISS default error page, which strikes me as being one heck of a missed opportunity. Any reason you can think of for not using a custom 404 that points back to the home page, a site index, a site search, something, anything other than a large boot with which to kick a potential visitor away?

Free marketing opportunities are few and far between at the best of times, so why aren’t more people using custom 404’s? Is it down to ignorance on behalf of the customer or the web design company?

Finally, what information would you suggest including on a custom 404 page?