If you're wary of using silly things like calendars to keep track of to-dos and reminders, Resnooze is worth checking out. This tool lets you schedule in weekly, daily, and monthly reminders to do something. Every time you get said reminders delivered to your in-box there are three simple options to get rid of it, or be reminded yet again--either a week or month from then.
Used reminders are set up in a small queue where you can tweak their frequency or get rid of them entirely. What's nice is that it will automatically register you for an account the first time you make a reminder, making it easy to come back later to add and remove planned reminders.
For now, Resnooze is limited to e-mail nags, so if you're looking for a more intrusive solution you might want to use Google or Yahoo's calendar tools, both of which include mobile SMS and instant-messaging reminders, although neither has the option to "snooze" a reminder for a later date.