This is the first add-on that really checks every link, including anchors inside the page.
It's also the first (and only?) add-on that checks a whole site. If you do that, the possibility to filter out some links is essential. (On my site I can filter .php to prevent getting lots of errors from my forum, for example.)
The possibility to open in a new window is also very handy. The page and the checker don't hide each other.
It's also the first linkchecker I've seen that let you go with just one click to the link that gives an error.
Really a very, very, very good extension!
I'm working on a system with Linux and this is the only simple good working linkchecker.
Everybody looking for a good linkchecker: this is the one you can use.