Alfred by Running with Crayons, Ltd., is a productivity application for your Mac. It functions as a super-fast launcher, a search engine, a file finder, and so much more. You can get the app for free or purchase an additional Powerpack for more functions.
If you need a reminder now and then, and if you want your reminders to appear across all your Macs and (soon) your iOS devices, then give Notificant a try. It's like a digital string around your finger.