fast 404 error

This is a continuation of the discussion here: http://groups.drupal.org/node/114124. The discussion stemmed from how much memory/cpu Drupal can take to provide a 404 error page (60MB+ if you have a decent module load).

While there have been several methods proposed for dealing with this, most are core hacks (though some have made it into Drupal 7). Most of the hacks deal with 404s for missing images/css/js files. They don't deal with other URLs that appear to be valid paths and not links to missing files.

Discussions referenced before the proposed module(s) were started on: