LinkChecker - LinkChecker is a free, GPL licensed website validator.
Ahrefs Backlink Checker - With the world's biggest index of live backlinks (over 15 trillion), Ahrefs Backlink Checker will show you the most complete backlink profile for any website.
DRKSpiderJava - DRKSpider is an open source website crawler (sitemap generator) and link checker. Features:
Backlinks by Neil Patel - A backlinks research tool for competitive intelligence
Pulno - SEO Audit and Website Analysis. Pull up your SEO.
Toxic Backlink Checker - Find out what's lurking in your backlink profile.