CronZilla enables you to setup as many url-cronjobs as you want. You can assign a time interval that you want CronZilla to call each of the urls specified. After saving your settings, CronZilla will follow your schedule as long as your FireFox browser is running and your internet-connection is up and available.
Developed by Stephan Helbig @ http://iStylr.com/
Plugin information page: http://iStylr.com/CronZilla/