Is there a broken-URL finder application that I could use to scan a website for broken links? (A Mac application would be better.)
For Windows users: Xenu
For *nix/Mac: wget (usually installed by default)
wget -r --spider -b -o /var/tmp/wget.log http://target.site.com/
-r- Recursive download
--spider- Check downloaded content for new links, then discard
-b- Run in the background
-o /var/tmp/wget.log- Target log file
http://target.site.com/- Replace with the site you want to check
wget will create a log file detailing the status codes reported while downloading content from the target site - note that this method will catch links to nonexistent content from within the domain, however, off-site links will not be checked (which is why it pays to monitor your site's error logs).
Google Webmaster tools will report on broken links. It's a simple tool, free to install to a website.....and platform agnostic. If you're not already using it, it can help you boost your SEO and useability as well.
This recursive HTML validator: http://htmlhelp.com/tools/validator/
should show you broken links among other errors. The tool can also be installed locally.
The W3C Link Checker is one of the oldest online services that checks for broken links.