incron - This program is an "inotify cron" system. It consists of a daemon and a table manipulator.

Reminduck - A simple reminder app made for elementaryOS with Vala and GTK. - matfantinel/reminduck

fcron - Fcron is a scheduler designed for Unix/Linux systems, especially usefull when your system is not up all the day long.

Memo to Me - Online email & SMS reminder service.

Anacron - Anacron is a periodic command scheduler. It executes commands at intervals specified in days.

Simply Remind Me - Free email and text reminder service so you don't forget the important things in life!