You're sat at your desk, or perhaps on the sofa, and you suddenly remember that it's your friend's birthday next Friday. Which means you need to buy a card on Wednesday. But that's 4 days away, so how are you going to remember?
One option is to use a really neat email service called Nudge Mail (www.nudgemail.com). Fire up your email program and create an email that says "Remember to buy a card". Send that email to email@example.com and the job's done. On Wednesday, you'll get sent a reminder email.
Needless to say, Nudge Mail doesn't just work for Wednesdays. Try sending a message to firstname.lastname@example.org if you prefer. Or to email@example.com. Or if you prefer, just send it to firstname.lastname@example.org and include the timing information in the subject line. Full details are available on the site.
My thanks to Geert for telling me about this. If there's a great freeware app or web-based service that you want to tell thousands of people about, just fill in the short form at http://www.techsupportalert.com/content/submit-product-review.htm and I'll try to write about your suggestion on these pages.