AJAX Date Picker is a free ASP.NET 2.0 calendar control that utilizes AJAX techniques to get data from the server without an entire page postback. You can specify the clickable dates on a given month as well as other days like holidays, etc. A great application for this control is for those sites that uses some reservation system to select valid dates. For example a travel site can use this control to render only valid dates. This controls limits the user on what dates to click. As a result you have a hybrid client/server validation.
This control is free along with the source code. Feel free to modify the code to suit your needs. If you want to change the look all you have to do is modify the Default.css file or create your own. Make sure you use the same class names. There is a property called ImageDirectory where you can specify the image directory (the default is images).