Free client side calendar control asp.net

Hello experts, up until now I have created a popup calendar using a little bit of javascript to call a window passing the textfield to an asp.net calendar control.  It works fine, but eats up server resources and I have problems with users minimizing the window, containing the calendar, and forgetting about it.  When they click the image to bring up the calendar it doesnt work, because the window is already open. I want to implement a client side calendar control that can interact with my asp.net textfields.  Does anyone know of a free control, or something I can implement strictly client side?

Here is what I am currently doing to simulate a popup calendar
'Page that calls calpopup.aspx page.
<script language="javascript">

//-----------Popup windows functions for calendars and cellnames
function start()
{
window.open('calpopup.aspx?textbox=txtsdate','cal','width=200,height=200,left=200,top=215')
}

function enddate()
{
window.open('calpopup.aspx?textbox=txtedate','cal','width=200,height=200,left=200,top=215')
}
</script>
<a onclick="start()"><img  id="imgstart" src="images\smallcalendar.gif" /></a>

'Popup calendar page codebehind
 Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'Put user code to initialize the page here
        control.Value = Request.QueryString("textbox").ToString()
    End Sub

    Protected Sub Change_Date(ByVal sender As System.Object, ByVal e As System.EventArgs)
        Dim strScript As String = "<script>window.opener.document.forms(0)." + control.Value + ".value = '"
        ' strScript += calDate.SelectedDate.ToString("yyyy/MM/dd")
        strScript += calDate.SelectedDate
        strScript += "';self.close()"
        strScript += "</" + "script>"
        RegisterClientScriptBlock("anything", strScript)
    End Sub