Day-O!

Fantastical is a flexible calendar with natural language event creation, a beautiful calendar and event list, support for CalDAV (including Google Calendar delegates) automatic alarms, and instant searching.

Itsycal is a tiny calendar for your Mac menu bar. If you want, it will display your appointments and serve as a companion to the Mac Calendar app.

Calendar 366 is the perfect menu bar mate for OS X's Calendar app. Click on the icon and get an overview of your events. Click on a date and see details, or switch to the expanded view* to see all upcoming events in just a single window.

Peek is a simple to use calendar for your iPhone.

Put a calendar on your menu bar, and access your calendar, weather, and more at a glance.