Update Scanner - Update Scanner is a Firefox extension to monitor web pages for updates.
Wachete - Track web page changes and get notified. Free Sign-up. Have all data in one place
WebRSS - RSS Widget - RSS to HTML - Display RSS for Free with Web RSS
WebSite-Watcher - WebSite-Watcher detects website updates and highlights all changes in the text. WebSite-Watcher monitors web pages, password protected pages, disucssion forums and much more.
Visualping - Visualping is the easiest to use website checker, webpage change monitoring, website change detector and website change alert software of the web. Read more about Visualping.
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.