I love Apple and they usually get things right, but the Calendar app leaves a bit to be desired. I often have to enter quick reminders and the built-in app can be cumbersome. For example, if I need to check the oven in one hour, I should be able to set a reminder to do that without having to tweak dials to set a clock for an hour from now. Why can't I just touch a button that says "+1 hour" and be done? To the rescue is 'Beep Me' or the longer name on the App Store, "Reminders and tasks made easy with Beep Me." I've been using Beep Me for several weeks and today's update makes the interface even nicer.
You can set alerts for a number of minutes/hours/days/months from the current time with the flick of a finger. You can even sync your alerts with DropBox or SugarSync, so if you run the app on one iOS device, like your iPad, that alert will be present on your iPhone, which you might be more likely to be carrying later.
As a developer, I've always tried to build apps that meet a specific need that I have, and then hope that others might have the same need. In this case, the developers of Beep Me have solved a problem that a lot of iPhone users have and they've done a good job. The newest version has a nice Post-It style theme and makes good use of the iPhone screen.