Anacron - Anacron is a periodic command scheduler. It executes commands at intervals specified in days.
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.
mcron - mcron is GNU's implementation of cron. It is a 100% compatible replacement for Vixie cron.
Simply Remind Me - Free email and text reminder service so you don't forget the important things in life!