VisualPing.io is a new web app that lets you monitor web pages and it sends an email alert when it detect any change in the content of the page. Unlike other web-based change detection tools, Visual Ping is easier to setup and completely free.
It takes 3 easy steps to monitor a web page as illustrated in this screenshot.
Enter the web address (URL) of a web page and Visual Ping will create a screenshot image of that page. You can create a selection anywhere on that page using your mouse and enter your email address. The service will monitor that particular area of the page every hour and whenever it detects a change, it will send you before and after screenshots of the specified area.
I have been testing the service for the last two days against several popular websites - including CNN and nytimes.com that are updated very frequently - and VisualPing.io does seem like a useful service. The email alerts arrive on time and the included before-n-after screenshots images are sharp and clear.
Google Docs does require some knowledge of XPath while in the case of Page Monitor, you can visually selected DOM Nodes on a page to add them to your monitor queue. The add-on will also highlight changes in pages that have been modified.