NETTUTS.com has posted this new tutorial you can use to monitor when and what pages people are trying to get to on your site that aren't found:

Although many web developers rely on server logs to keep an eye out for hits on error pages, I'm going to take a different approach by using a PHP generated email. In addition, we will spice up the design a bit, add basic navigation and link to the website sitemap.

He briefly explains what error (404/403) pages are and how they work before getting into the mailing portion - a script that, using the phpMailer tool, notifies you when people hit the 404/403 page and includes things like their IP, where they came from and what the requested page was.