404 Redirect

You not only want to be aware of broken links on your site but you want to know about links on other sites that lead to old or non-existent pages on your site. You can do this by keeping an eye on your logs and looking for those coming in getting page not found errors. With this you can do three things:
1) Find out what pages are attempted to be accessed the most. If people are coming to a recently deleted page you either want to put something back up in its place or implement a 301 redirect from that page to the page that is the closest match.
2) Find out where the traffic is coming from. If other sites have links to you that don't work sometimes you can get that corrected by simply asking them to fix the link. They linked to you for a reason and it likely is in their audience's best interest to make sure the link works.
3) Implement a custom 404-redirect page. You won't be able to fix or redirect every broken link coming to your site but you can make sure that anybody coming to your site from a bad link gets more than the generic white "page not found" screen. By creating a customized page visitors can be redirected to relevant content on your site, you will keep more visitors and have an opportunity to convert them. You can read a longer tutorial on 404-