Fantastical is a front-end to your calendar program (either iCal or BusyCal) that's available instantly from the menu bar via a click or a keyboard shortcut. I use it everyday, many times a day, and it's an indispensible part of my daily routine.
My use case Fantastical goes like this: someone asks if I'm available on a given date, I type Control-Option-Space and a mini-version of my calendar reveals from my menu bar. I quickly scan my calendar from this UI and click on a given day to see what I've got scheduled that day. Color-coding helps make quick sense of my various calendars.
But, it's the second part where things get interesting.
If I am available on a given day, I can type sentence like "Lunch with John at 123 Main Street on Tuesday" into the text field at the top of Fantastical and it translates my sentence into an event and adds it to my calendar. Using iCal, this would involve lunching the app and about two dozen additional clicks to find the date and add the details of an appointment. Fantastical's natural language engine is extremely impressive and alone worth the $19.99 price of admission. The icing is that Fantastical supports CalDAV sync allowing it to sync with MobileMe, Google Calendar, and Yahoo! Calendar. Fantastical is available for $19.99 from the Mac App Store or directly from the Flexibits Store. Fantastical requires Mac OS X 10.6 or later. Fantastical 1.0.2 is a free update for all existing customers.