Google Webmaster Tools is a free service that provides a wealth of information directly from Google. Once you have verified a site with Google, they’ll give you access to all sorts of information.
Here are just a few features of Google Webmaster Tools:
Google Webmaster Tools will show all sorts of errors with a site. Not only does it show broken links on the site, but also links that are driving traffic to the site for which there is no valid page. Google even tells you pages it knows about but has been restricted from crawling. That’s good to know incase someone accidently blocks to much.