I have posted a solution before for an Ajax calendar with drop downs.But there is a control called CalendarExtenderPlus .you can find it from http://lionsden.co.il/codeden/?p=222 .This control has developed by Hanan Schwartzberg.But there were some issues.Anyway Hanan could fix some of the issues.But I noted there was an issue when we are using this control with other Ajax controls.The issue was can not select a date after the first selection.Because calendar control is getting stuck.We were sending more than 30 messages to each other to figure out the issue.Anyway finally I could find the issue.That was 2 java script errors.
The error was
Microsoft JScript runtime error: Sys.InvalidOperationException: Type Sys.Timer has already been registered.

Anyway I found this solution for this.You should modify the ScriptManager code like this.Then everything will work fine.Thank you very much for your great work Hanan.I just put pepper and salt on to it.





        runat="server"
        EnablePageMethods="true"  
        EnableScriptGlobalization="true"  
        ScriptMode="Release" />


I recommend to install AJAX Control Toolkit May 2009 Release or later version like Version 3.0.30512 for best result with this.