You have it in your browser - most of the browsers now in use can notify when a bookmarked page has changed.

The problem with this approache is that most of dynamic pages have in one place or another the date or/and current time so every time you download them they are automatically changed. To cope with that problem you need a mechanism for recognizing such trivial changes and that is not trivial - but I am currently writing a web application for that exact task: Active Bookmarks. You can download the code, it is GPLed.

Additionally at Yet another HTML diff just a few days ago I posted the code of the heart of the application.