I suck at remembering when stuff is supposed to happen. So when I moved my e-mail service to Google Apps, I got the calendar functionality with it.
What I just found the other day is that you can have it send you a text message at some specific time before an appointment. So for fun, I had it send me a reminder that I'm having dinner with Stephanie tonight. And what do you know, there it was on my phone three hours prior. Because I'm just the kind of person who would realize that driving home was not the right direction.