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.
You could also try A1 Website Analyzer (Windows tool, but can run in VMWare/Fusion under Mac) It can tell you of all URL references site-wide (e.g. which pages link to a 404 URL + usually also line numbers), HTML/CSS errors etc.
The W3C Link Checker is one of the oldest online services that checks for broken links.