Right now, I am involved in some web designing work involving a very huge website, with a large number of links. I was going through them one by one to check whether the links are working until I thought "Hey! There must be an easier way to do this."
And there is. My latest favourite Firefox extension is the LinkChecker 0.6.1.
Just click the button and all live links show up highlighted in Green while dead links, in Red.
Really useful tool.