Dynamically populate a checkbox list control
[WebMethod]
public GetUsersResponse[] LoadUsers()
{
    if (HttpContext.Current.Session["Users"] != null)
    {
        return (List<GetUsersResponse>)HttpContext.Current.Session["Users"];
    }

    return new List<GetUsersResponse>();

}

[WebMethod]
public GetUsersResponse[] GetUsers(string query)
{
    var users = new List<string>
    {
        "Brad Pitt",
        "Brad Pitt2",
        "Brad Pitt3",
        "Angelina Jolie",
        "Jeniffer Aniston",
        "Tom Cruise",
        "Katie Holmes",
        "Tom Hanks",
        "Sean Pen",
        "Jude Law",
        "Bruce Willis"
    };

    var returnUsers = users.Where(s => s.ToLower().Trim().StartsWith(query.ToLower().Trim()))
                     .Select(s => new GetUsersResponce { Name = s })
                     .ToArray();

    HttpContext.Current.Session["Users"] = returnUsers;
    HttpContext.Current.Session["Query"] = query;
    return returnUsers;
}

public class GetUsersResponse
{
    public string Name { get; set; }
}

<script type="text/javascript">
    $(function () {
        $.ajaxSetup({ type: 'POST', dataType: 'json', contentType: 'application/json', data: {} });

        $.ajax({
            url: 'Lookup.asmx/LoadUsers',
            data: '',
            success: function (data) {
                var responseJson = data.d;
                if (responseJson.length > 0) {
                    $.each(responseJson, function () {
                        $("#result").append("<li><input type='checkbox'>" + this.Name + "</input></li>")
                    });
                }
            }
        });



        $("#txtType").keyup(function () {
            var input = $(this).val();
            $("#result").html("");
            if (input && input.length > 3) {
                $.ajax({
                    url: 'Lookup.asmx/GetUsers',
                    data: '{ "query": "' + input + '" }',
                    success: function (data) {
                        var responseJson = data.d;
                        if (responseJson.length > 0) {
                            $.each(responseJson, function () {
                                $("#result").append("<li><input type='checkbox'>" + this.Name + "</input></li>")
                            });
                        }
                    }
                });

            }
        });
    });
</script>
<table cellpadding="5">
    <tr>
        <td>Search</td>
        <td><asp:TextBox ID="txtType" runat="server" ClientIDMode="Static" /></td>
    </tr>
    <tr>
        <td colspan="2">
            <ul id="result" style="list-style: none;">
            </ul>
        </td>
    </tr>
</table>