Fcron is a periodical command scheduler which aims at replacing Vixie Cron, so it implements most of its functionalities.
But fcron does not assume that your system is running neither all the time nor regularly : you can, for instance, tell fcron to execute tasks every x hours y minutes of system up time or to do a job only once in a specified interval of time.
Fcron has also much more functionalities : you can also set a nice value to a job, run it depending on the system load average and much more!
(please see http://fcron.free.fr -- published under the GPL license)
Previous home of the official fcron sources repository -- now moved to https://github.com/yo8192/fcron.