I found out about this webmaster tool from a fellow member of the DC Web Women email list. LinkPatch (free for use on one website, fee for service for multiple sites) monitors your website for errors and reports them to you.
To use Linkpatch, you place code on your website 404 error page. You then get an email report as soon as someone reaches your 404 error page. The tracking code records the information needed to find and fix the problem.
Sounds like a nifty tool. Would love to hear from anyone who uses this.