BostadsBotten is a project mainly written in Shell, it's free.
Simple bash script matching two files and sending an email in case they don't match
A very simple script pulling a website from the internet and matching it against a previous sample. If there's a mismatch suggesting that an appartment has been listed an email will be sent.
It depends on curl, which can be replaced with about anything you like, and a sendmail compliant MTA.