There does seem to be an opportunity here. The free tools I've used for doing this all hit the web server and are either command-line based (wget, etc.) and/or ancient: "Xenu's Link Sleuth" for Windows. ( http://home.snafu.de/tilman/xenulink.html )

I'd love to hear more about what other people are using, particularly if tools can find broken JavaScript/AJAX links.