I have just found this good blog post that tells you how you can monetize your 404 error pages. 404 pages often have a lot of hits, but they usually do not have any ads on them.
http://www.monetizemyweb.com/make-money ... pages.html

thats a great idea...

i use custom 404, and 500 errors too

The Xenu SEO Tools can help u to check the 404 links & images.