Munin the monitoring tool surveys all your computers and remembers what it saw.

Monitorix is a free, open-source, lightweight system monitoring tool designed to monitor as many services and system resources as possible.

HTTP(s) status dashboard webpage via shell script

see what changed with SiteDelta Highlight and SiteDelta Watch

Scan web pages for updates.

Monitors a number of web pages and tracks changes based on the content of the web pages. Allows to monitor several protocols, including HTTP and HTTPS. Allows to view and record differences.