program that scans a website for changes and emails me when the website changes.

I need a program that will scan a certain part of a website and check every 30min or another time and email if the area that it scanned changes. It needs to scan one website online. It needs to email if there is any errors. the program will need to start scanning hundreds of different URL's as we add more URL's to scan, so up to thousands. It needs to be able to scan relatively fast if we are scanning thousands.