Mac (Mountain Lion): There are lots of different ways to remind ourselves to do things, including to-do lists, task lists or calendars. But if you're looking for something much simpler that just reminds you to do something in a specified period of time, Scheduled Notifications does the job.
Scheduled Notifications sits in your menubar and works with Notification Center as a lightweight system to give yourself quick reminders. It's not meant as a to-do list. Scheduled Notifications is meant more as a disappearing sticky note that reappears when the time is right. You can set Scheduled Notifications to notify you at a specific time, or just type in something like "in 20 minutes".