Crontab UI - Easy and safe way to manage your crontab file
Reminder Commander - Reminder manager that reminds you about important events in advance.
Scheduled tasks - GNOME GUI for configuring a users' cron (automatic jobs). Some of its features are:
Memo to Me - Online email & SMS reminder service.
Surge for Mac - Advanced Web Debugging Proxy for Mac & iOS
Kana Reminder - Kana Reminder is a program which can be used to set a reminder to be triggered at a specified time.