Alarms app is the reminder system you never knew your Mac was missing
It's extremely easy to get started with Alarms. Everything is drag-and-droppable, and it feels like it should be a built-in part of OS X. Here's one example of Alarms' UI slickness: When you drag down on the Alarms menubar icon to a set a timer, the duration is based on the length of your drag. You can also set notifications for a specific time using the drop-down calendar view, and even drag a URL from your browser to the calendar to create a new alert.
Alarms is in beta right now, so it's free to download and use. It will eventually cost money, but the developers haven't set the price yet. In other words, now's the time to grab this app and take it for a spin.