One of the new capabilities Mango gives to developers is to schedule reminders.
One Thing is an app by Max Lieblich which uses this facility to make sure you never forget that one very important thing.
The app will schedule up to 20 repeat reminders for the same item at random times with an early or late bias. According to Operand Conditioning theory, intermittent reinforcement is the strongest tool for making sure that the item will remain in the front of your brain.
The app also lets you post your reminder to Facebook, so your friends will also serve as a backup for your fallible noggin.
One Thing is $0.99 with a free trial and can be found in Marketplace here.