Since Q2 2010 all new functionalities are listed in the control's Release Notes posted here.

RadCalendar and RadDatePicker/RadDateTimePicker/RadTimePicker controls have new animation type Slide in addition to the default Fade animation. To enable it, you can set the Type property in the particular control ShowAnimation and HideAnimation settings. Find more information here.

CopyASPX

<telerik:RadCalendar ID="RadCalendar1" runat="server">
    <FastNavigationSettings>      
    <ShowAnimation Type="Slide" />
        <HideAnimation Type="Slide" />
    </FastNavigationSettings>
</telerik:RadCalendar>
<telerik:RadDatePicker ID="RadDatePicker1" runat="server">
    <ShowAnimation Type="Slide" />
    <HideAnimation Type="Slide" />        
</telerik:RadDatePicker>

With its Q3 2009 version RadDatePicker/RadDateTimePicker/RadCalendar expose integrated fade-in/fade-out JQuery animations on fast navigation and popup show/hide. See this article for details.

With its Q2 2009 version RadDatePicker and RadDateTimePicker expose two new features - screen boundary detection for their popup elements and explicit setting for expand popup direction. Review the two online demos linked below for details:

RadDatePicker - First look demo

RadDateTimePicker - First look demo

RadCalendar rendering is now changed to use accessible tables and CSS sprites. All skins are reworked for the new rendering.

RadCalendar now supports binding to List.

RadTimePicker supports binding to custom collection of DateTime.

There are no major new features introduced in RadCalendar for ASP.NET AJAX Q2 2008 - merely enhancements/optimizations are included.