Calendar Extender Pros/Cons
The Calendar Extender is pretty neat in how it works, as part of the AJAX Control Toolkit. By clicking on a control or button, it renders a calendar, which has a link to today's date at the bottom, the calendar (and other month days) in the center, and a header on the top. The header allows you to click it, transforming to a year view, which you can select a specific month, or you can even navigate down to a decade view, which allows you to select a specific year (you can see a demonstration here).
It's really neat in how it works, and is very functional, but there are a few issues I find with it. For instance, when using an image as the button, most people don't realize that you have to click the button again to make it go away. Most people are used to a little x in the upper right corner, or being able to click away somewhere else and that disables it. In addition, I haven't found a way to disable dates passed the current day with it, or do other date processing tricks.
Still, it's overtly useful; however, it's limitations must be understood. If you can do any of these things and I am in error, please provide a comment.