Google have added a piece of useful functionality to their Webmaster Tools console that shows you links pointing to your site that cause 404 page not found errors.
Sometimes a site will link to you and later you may remove the page that the link targets. Or you may change the URLs within your site and forget to link to the new one from some pages. These are the two common problems that Google is trying to help webmasters fix in a straight forward manner. Broken links that return an error to the user is poor for usability, and you are also missing out on link equity from links that point to non-existent pages, meaning your SEO efforts are not optimal.
The Webmaster Tools dashboard has a clear list of URLs that generate 404 errors, showing the date the error was detected and a link to the URLs that are causing a 404 error. The list of URLs can then be downloaded into CSV format.
In order to fix the errors, you could fix broken links within your own site and contact site owners of external sites and ask them to change their link. In order to ensure the flow of link equity into your site and strengthen your search rankings, you should also issue 301 redirects from the redundant URLs into a relevant equivalent.
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