Google Webmaster Tools has just released a great new feature that allows you to find out which sites are linking to non-existant (i.e. 404) pages on your website.
Take a look at the example below for this website:
You can find the information in Webmaster Tools by clicking on Diagnostics > Web Crawl > Not Found. This screen will tell you how many people link to each non-existant page. If you click the figure, it will list all the URLs that link to each page. As with a lot of the data available in Webmaster Tools, you can also download it.
Redirecting from these non-existant URLs to ones that do exist is a great way of boosting the number of inbound links to your website. Links that point to non-existant pages will not help you rank in the search engines. Using a 301 redirect to point these links to a real page will help your rankings. Matt Cutts describes these as “free links to your site.”
Previously, it was very difficult to find the sites that linked to non-existant pages. This is a great addition to the webmaster tool collection and will become a very valuable resource for webmasters to increase the number of links to their site.