Here's a better solution: Free service TextMemos sends scheduled reminders to your phone, inbox, or Twitter account. It's a good replacement for the PingMe service, which just announced it's closing its doors.
After registering for an account (a process that wisely requires validation via e-mail), just click Add Reminder, then fill in the details: the message you want to send, how and when you want it sent, and who should get it (TextMemos lets you direct reminders to yourself or someone else, which is definitely handy).
In my initial testing, the service seemed a little buggy with regard to time. I'd set a reminder, then click View Reminders to double-check the settings, and the times shown would be totally off. The good news is the reminders themselves arrived at the proper times, and that's ultimately what matters.
TextMemos may not be the prettiest service in the world, and it's annoyingly ad-heavy, but it definitely fills the void left by PingMe. If you want a quick, easy, and free way to send yourself (or someone else) a reminder, this is it.