Responsive Viewer - View multiple screen dimensions in one view
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.
Responsive Website Test - Test any website on desktop, mobile, and tablet
Distill Web Monitor - Distill is a web monitoring tool. It can monitor RSS feeds, a webpage or a part of webpage. Alerts in the form of pop-up, audio or emails can be received.
Multi Screen Test - Test your site at different screen resolutions & devices.
Update Scanner - Update Scanner is a Firefox extension to monitor web pages for updates.