Filewatch is a small PHP script that keeps an eye on the files on your web site, and notifies you by email if any files are added, deleted or changed.
Because it runs on the server, the script is not limited to just checking web pages - it can also monitor hidden and system files in your hosting account.
There are several ways you can implement this script. For example, it can be run as a cron job, included invisibly in a web page, or just a standalone script that you manually check.
Filewatch has saved my bacon several times, by helping to identify changed files after a security exploit on clients' sites.