uses 4.0 .Net Framework and AjaxToolkit v4

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="ajaxToolkit" %>

 <ajaxToolkit:ComboBox ID="cbProducts" runat="server" DropDownStyle="DropDownList"
                                    AutoCompleteMode="Suggest" RenderMode="Block"  AutoPostBack="true" OnSelectedIndexChanged="ComboBox1_SelectedIndexChanged"
                                    AppendDataBoundItems="false" CssClass="AquaStyle">
                                </ajaxToolkit:ComboBox>

code behind example

private void FormatPage() {

 WebServiceSoapClient wsc = new WebServiceSoapClient ();

cbProducts.DataTextField = "itemNo";

cbProducts.DataValueField = "itemNo";

cbProducts.DataSource = wscc.GetProducts(); //dataset could use generic or other bindable object

cbProducts.DataBind();

cbProducts.Items.Add("--Select Item --");  //add item to top of list

 cbProducts.SelectedValue = ""; 

}

link to more examples here