The best 404 page helps the user find what they were looking for. You can run a cron job on your server logs to analyze your log files for statistically significant problems (not just 404's); that script can report them all to you my email or SMS or whatever. Further, you can run cron jobs that crawl your site looking for broken links, and/or replay some of the previously-working GET requests; then you will find broken links even when nobody is using your site.