Checking out CronniX:
CronniX is a Aqua frontend to the powerful Unix tool "cron". Cron is a Unix system service that allows scheduled execution of scripts, programs, applications - in short anything that can be started from the command line. This includes OSX applications and AppleScripts
It's surprisingly full-featured, including the ability to import & export existing crontabs and a 'Run Now' command to test tasks, along with fairly robust documentation.