We recently re-designed Developer Fusion and as part of that we needed to ensure that any external links were not broken in the process. In order to monitor this, we used the awesome LogParser tool. All you need to do is open up a command prompt, navigate to the directory with your web site's log files in, and run a query like this:

"c:\program files (x86)\log parser 2.2\logparser" "SELECT top 500 cs-uri-stem,count(*) FROM u_ex*.log WHERE sc-status=404 GROUP BY cs-uri-stem order by count(*) desc" -rtp:-1 > topMissingUrls.txt

And you've got a text file with the top 500 requested URLs that are returning 404. Simple!