Lightning Calendar - Organize your life — it’s about time!
Simply Remind Me - Free email and text reminder service so you don't forget the important things in life!
GMinder - GMinder is an open source Windows System Tray app that pops up to remind you about Google Calendar events.
FollowUpThen - The simplest way to schedule an email reminder
Calendar Lock PEA - Calendar Lock PEA is an encrypted desktop calendar with a day, week and month view. The shown calendars are never stored unencrypted on your disk, but exist only in the RAM. Platform-independent, Open Source, no installation or registration required.
Auto Mail Sender - Auto Mail Sender Standard Edition official page. A powerful and easy-to-use eMail scheduler.