If you’re a Webmaster or site owner, this should be of interest to you!
Within Google Webmaster Tools, it has been possible for a while to get a list of those pages that Google has as ‘Not found’ or 404.
First and foremost, if you have pages not found, you should take steps to fix them, e.g. if a page has been renamed, redirect the old page to the new page (301 permanent redirect).
Now though, Google Webmaster Tools shows you where your 404 pages are being linked from (Diagnostics | Web crawl):
Webmaster Tools Shows Inbound Links for 404 Pages
Once you analyse the inbound links to old or broken 404 pages, you can take the appropriate steps to fix the issue. You could contact the Webmaster of the site that is linking to yours and ask them to update to give their visitors a better experience or you could 301 redirect the old page to new page if there is a suitable replacement.
Whichever route you take, you should definitely keep an eye on your Webmaster Tools account and particularly your diagnostics section. Keep your site in order and you may just get rewarded for it by gaining additional traffic.
Webmaster Tools Shows Crawl Error Sources
Matt Cutts Tutorial on Using Web Crawl Diagnostics