404s "tracker" for Mac OS X I'm looking for OS X software that can help me track 404 errors on my site.
What are you using to watch the stats on your website? Most stats packages that read the logs will give you a 404 report (eg. AWStats, Summary).

If you're looking for a program to run on your Mac to check that for you; Piltdown Link Checker, Integrity, WebLight or LinkByLink are all options.