MenuMinder is a simple piece of software that does one thing, but does it reasonably well. It sits in your menubar and allows you to enter quick reminders to yourself. The reminders pop up whenever you like (though in an unfortunate brushed metal interface, right). You can also set them to be emailed or sent to your phone via SMS.
While you can obviously set events in iCal or even use Quicksilver for quick reminders, MenuMinder is extremely quick and straightforward. If, like me, you're often writing little reminder notes to yourself this might be just the ticket.
MenuMinder is $10 and a demo is available.