Rainlendar - Rainlendar - Customizable desktop calendar
Reminduck - A simple reminder app made for elementaryOS with Vala and GTK. - matfantinel/reminduck
Fantastical 2 - Fantastical, the Mac calendar app you'll enjoy using. Quickly create new events with natural language input and more.
Memo to Me - Online email & SMS reminder service.
Date Reminder - Date Reminder reminds you of recurring or nonrecurring events, like birthdays, bills to pay...
Simply Remind Me - Free email and text reminder service so you don't forget the important things in life!