If you have a site with a lot of links and don’t have the time to continually be checking whether or not they are still valid, then you need the W3C’s Link Checker tool. This online tool parses your site’s source and validates each link you reference and notifies you of:

• Invalid/broken links
• Links which reference a site which has moved (redirects)
• Errors in your anchor (<a>) tag

While the online version I link to above is good for a quick crawl of your site, there is a downloadable program you can use which gives you more options and detail. Either way, both are a fine tool any webmaster should know about.