Google recently added a useful new feature to their Webmaster Central portal, which Google employee Matt Cutts says can help you get some extra links. It allows you to see dead URLs that sites are linking to on your site (a.k.a. pages that don’t exist, and return 404 pages). Essentially, after getting a list of those URLs, you can set up some good (and search engine optimized) content at those URLs. Voila, extra links.
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”.
Also, BloggingTips.com has a more in-depth post on making use of the tool.