Dramatically speeds up 404 handling. As of this writing, there is a bug when using drush. See this issue for a patch: https://drupal.org/node/2114479 . Thanks to Dan Shumaker for this tip.
Here's the project page description:
Drupal has expensive 404 errors. On an 'average' site with an 'average' module load, you can be looking at 60-100MB of memory being consumed on your server to deliver a 404. Consider a page with a bad .gif link and a missing .css file. That page will generate 2 404s along with the actual load of the page. You are most likely looking at 180MB of memory to server that page rather than the 60MB it should take.
That's where Fast 404 comes in. This module combines a very common method of handling missing image/file 404 errors (discussed here and planned for Drupal 8) with a method created by dpardo (a co-maintainer of this project) to deliver super fast 404 error pages for both missing images and bad paths. Depending on which method of implementation you choose (aggressive or super aggressive) you can deliver 404 errors using less than 1MB of memory on your server.
Drupal 7 Core Updates:
Drupal 7 core has updated to add a rudimentary version of what this module implements. It allows you to set an excluded set of paths, a list of extensions to Fast 404 on, as well as the plain HTML that is delivered.
See the project page to read more: https://drupal.org/project/fast_404