OSX: launchd (and Lingon, a nice GUI for launchd)
In Mac OS X v10.4 Tiger, Apple introduced a new system startup program called launchd. The launchd daemon takes over many tasks from cron, xinetd, mach_init, and init, which are UNIX programs that traditionally have handled system initialization, called systems scripts, run startup items, and generally prepared the system for the user. And they still exist on Mac OS X Tiger, but launchd has superseded them in many instances. These venerable programs are widely used by system administrators, open source developers, managers of web services, even consumers who want to use cron to manage iCal scheduling, and they can still be called with launchd.
Playing with launchd in the terminal is not for the faint hearted, so Lingon (from the creators of the well known OSX text editor Smultron) is a very nice GUI that allows anyone to set and manage jobs! 🙂