Fast 404 Module

404 Errors are expensive in Drupal. A 404 page load can consume 60MB+ RAM just to tell someone there is nothing there. What's worse, if the 404 is an image, js or css file, you have just spent 60MB+ to serve a page to a browser which will only read the header.

This module (which was inspired and started by dpardo on Drupal.org--and also incorporates some features discussed in other threads) serves fast 404 errors either out of the settings.php file (if you are brave) or during bootstrap.