Usability 
Google's launched a mobile version of Google Calendar. The mobile url is google.com/calendar/m. It's elegantly simple both in ease of use and functionality. The default (and only view) is of one month starting with today. Prev and Next links jump a month back or forward. Of course the mobile and desktop web versions of Google Calendar are always in sync.
The Quick Add feature, which is also part of the desktop web version of Calendar, is very slick. Google recognizes natural language phrases, like:
"Staff meeting next Monday at 13:00"
"Vacation 9/23 - 9/26" - creates a recurring event.
Or even "Dinner at Mom's on the first Tuesday of every month"
Pretty amazing. There's more on Quick Add here.