WebListener is a small, quick, open-source PHP application that allows you to monitor one or more websites by configuring a single XML file. The release version will allow you to track up-time and receive site outages via RSS.

Please note: Flag icons from famfamfam are used under a Creative Commons Attribution 2.5 License. Globe and warning icons are from http://art.gnome.org/ until I find time to create better icons.