There have been many useful and interesting add-ons to Firefox to increase usability. I just recently discovered another one: a free link checker from Kevin A. Freitas. It’s a great way to check your link validity, but one page at a time (which can get a little tedious).
For smaller sites this is a great option, but for larger sites you’ll probably need to go with something with more firepower, like Web Link Validator. There’s a “lite” version (which is free) and a heavy-duty professional version (with a heavy duty price tag!).
For Wordpress there a nice little plugin called Broken Link Checker. With this plugin you can check your links for any breakage from the back end admin panel.
And for anyone who doesn’t know about the relevance of broken links check out Google’s word on link checking and broken links. Webmaster Tools does list any crawl errors that come up for your site, but depending on the kind of error it may not show you the source page of the error. Frankly, I don’t think this is very helpful to list an error and then not give an indication of where to find it on your site.
If you have other suggestions for free ways to check you site’s links please let me know. I’d love to add to this list.