I've posted a wrapper ASP.NET around the jQuery.UI Datepickercontrol. This small client side calendar control is compact, looks nice and is very easy to use and I've added it some time back to my control library.
This is primarily an update for the jQuery.ui version, and so I spend a few hours or so cleaning it up which wasn't as easy as it could have been since the API has changed quite drastically from Marc's original implementation. The biggest changes have to do with the theming integration and the resulting explosion of related resources.
If you want to use this component you can check it out a sample and the code here:
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