No matter how pretty a calendar interface may be, it's always easier to just tell the calendar, in plain English, what you want. QuickCal makes it really easy. Instead of flipping through days, weeks, and months in iCal, BusyCal, or Google Calendar, you can just tell QuickCal the what, when, and where. It takes care of the rest.
They've got an iPhone app, too. So much better than spinning a wheel a la Showcase Showdown every time I need to pick a date.