SEO PowerSuite - Get all SEO tools in one pack - download free edition of SEO PowerSuite and get top 10 rankings for your site on Google and other search engines!
BrokenLinkCheck - Free Broken Link Checker / Online Dead Link Checking Tool - find bad hyperlinks
Moz - Backed by industry-leading data and the largest community of SEOs on the planet, Moz builds tools that make inbound marketing easy.
Dead Link Checker - Check for broken links on any website
SEO Tools Centre - SEO Tools Centre provides SEO checker services for websites.
LinkChecker - LinkChecker is a free, GPL licensed website validator.