For years webmasters have been able to view the URLs on their site that are linked from various websites but actually don't exist. Today webmasters can fix these broken links through Webmaster Tools Crawl error sources. Crawl error sources displays the source URL containing the broken link to their site -- no matter where it exists on the web.
What about any "file not found" URLs that haven't yet been fixed? Let's improve that, too: Webmaster Tools provides site owners with the "file not found" widget, (aka the 404 widget). It's a small snippet of code that can change the message from this unfriendly one:
To this helpful message (note the handy URL suggestion, search box, and link to existing sitemap and parent directory):
So, if you're a site owner, help us help you fix broken links and make your "file not found" pages more useful by signing up for Webmaster Tools. Here's to an even better experience on the web!
Posted by Maile Ohye, Developer Programs Tech Lead