The best reminders are the ones that you don't have to go too far out of your way to set. To that end, ReminderBaba makes it dead simple to remind "future you" to do something. Simply add one of the service's many custom email addresses and automatically get a reminder later.
ReminderBaba has a number of useful email formats you can use, including a set number of minutes from now, a particular day of the week, or even recurring reminders. For example, if you want to be reminded of something every month on the 1st, send an email to "[email protected]". You can also see a list of all the reminders you have waiting by emailing "[email protected]". The service doesn't require you to install anything or even create an account. You can start adding any of the email address formats from this list to your emails right now.