If you use iCal, QuickCal is a great widget.
Just launch Dashboard by pressing F4 (or whatever your shortcut is) and type something like the following into the QuickCal text field:
"Dinner w/ Susan next wed from 7pm to 11:45pm"
QuickCal automatically extracts the relevant pieces of data and creates a new event in your iCal calendar. It's an impressive little application that gave me a reason to dust off Dashboard.
Mac OS X is great and everything, but iOS is even better. QuickCal Mobile ($0.99, iTunes) is companion app for iOS that allows you to create calendar events by typing them into your device. Now that's cool.
Tip: Ryan Kaplan