Broken Link Checker

I have found this link to be invaluable:

BROKEN LINK CHECKER

This eliminates the need to purchase an extension that does the same.  It also appears to ignore certain portions of a website, such as a calendar which could go on infinitum.   You ask what this has to do with SEO?   The last thing you want on your site is a broken link.  It shows an unprofessional site, one of the first signs of an amateur site, along with inconsistency of design.   Yes, this affects your search engine optimization.