I can’t believe a new feature from Google isn’t getting more notice, because it converts already-existing links to your site into much higher quality links, for free. The Google webmaster blog just announced that you can find the pages that link to 404 pages on your site.
Let me back up and give you a little history. When someone comes to your site’s webserver and asks for a page that doesn’t exist, like http://www.mattcutts.com/asdfasdfasdf , most web servers are configured to return an HTTP status code of 404, which means that the page was “Not Found.” If someone links to a page on your site that doesn’t exist, most webservers give a pretty sucky experience: visitors usually land on a pretty useless page, and search engines might not give you full credit for those 404 errors.
Now Google’s webmaster portal lets you see who is linking to your 404 pages. Once you register your site, click on Diagnostics, then Web crawl, and select “Not found”. You’ll see something like this:
