I’ve updated my 404 Notifier plugin to make it look nicer in WordPress 2.5 (seems like old admin CSS classes were removed).
This plugin logs 404 hits on your WordPress powered site and will notify you of them via e-mail or in an RSS feed.
The download and more information are available on my WordPress Plugins page.
If you have any trouble with this, please open a thread in the WP Support Forums and send me the link.
UPDATE: The RSS feed was broken in version 1.2, version 1.2a is now available to fix this.