incron
This program is an "inotify cron" system. It consists of a daemon and a table manipulator. You can use it a similar way as the regular cron. The difference is that the inotify cron handles filesystem events rather than time periods.
mcron
mcron is GNU's implementation of cron. It is a 100% compatible replacement for Vixie cron.
Anacron
Anacron is a periodic command scheduler. It executes commands at intervals specified in days. Unlike cron, it does not assume ...
fcron
Fcron is a periodical command scheduler which aims at replacing Vixie Cron, so it implements most of its functionalities.