Update Scanner is a Firefox addon to monitor web pages for updates. Useful for websites that don't provide Atom or RSS feeds.

WebSite-Watcher detects website updates and highlights all changes in the text.

ChangeDetect is a webpage change monitoring platform.

Distill runs in your browser to check monitored pages for changes. Get instant alerts as soon as a change is detected.

ChangeMon.Com is an online website change monitor service.