LinkChecker - LinkChecker is a free, GPL licensed website validator.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
DRKSpiderJava - DRKSpider is an open source website crawler (sitemap generator) and link checker. Features:
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Dead Link Checker - Check for broken links on any website
Better Uptime - We call you when your website goes down