Timeframe, a better calendar
Timeframe is "Click-draggable. Range-makeable. A better calendar.", and is open-source. It is thoroughly impressive, a great implementation, and works best in Safari!
The demonstration (pictured) is by far the best calendar "widget" I've seen in a browser. One of my favorite things is the date range tools which is where Timeframe strived to excel and succeeded. Not only can you enter in a date range by typing it in, which will then result in the same range being selected in the calendar view above, but you can also click, drag, and select the range within the calendar view itself.
There are some desktop application calendars that pale in comparison to Timeframe. iPhoto is the first example that comes to my mind - you can't even select a date range in iPhoto! Anyways. Give Timeframe a spin and if you're a developer, start putting this into your applications pronto.