incron!

nnCron is an advanced and very powerful scheduler, scripting tool and automation manager. nnCron is very small in size (900k), but its capabilities are truly colossal. Besides possessing the traditional scheduler features (starting programs, displaying reminder messages and opening documents at specified time)

Anacron is a periodic command scheduler. It executes commands at intervals specified in days. Unlike cron, it does not assume that the system is running continuously. It can therefore be used to control the execution of daily, weekly and monthly jobs (or anything with a period of n days), on systems that don't run 24 hours a day. When installed and configured properly, Anacron will make sure that the commands are run at the specified intervals as closely as machine-uptime permits.

Fcron is a periodical command scheduler which aims at replacing Vixie Cron, so it implements most of its functionalities.

mcron is GNU's implementation of cron. It is a 100% compatible replacement for Vixie cron.