<input type="text" value="" id="ip1" class="ip1" />
<input type="button" value="Add" class="bt1" id="bt1" />
<br/>
<select>
    <option value="volvo">Volvo</option>
    <option value="saab">Saab</option>
</select>

$(document).ready(function (e) {
    var $txtVal = $('#ip1');
    $(".bt1").click(function () {
        var opt = $("#ip1").val();
        if ($txtVal.val()) {
            $('<option />', {
                text: $txtVal.val(),
                value: $txtVal.val()
            }).appendTo('select');
        }
    });
});

<input type="text" value="" id="ip1" class="ip1" />
<input type="button" value="Add" class="bt1" id="bt1" />
    <br/>
   <select>
     <option value="volvo">Volvo</option>
     <option value="saab">Saab</option>
    </select>


$(document).ready(function () {
    $(".bt1").click(function () {
        var opt = $("#ip1").val();
        if (opt != "" && opt != " "){
            opt = "<option value=" + opt + ">" + opt + "</option>";
            $("select").append(opt);
        }
    });
});

<script type="text/javascript"        src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
    <script language="javascript">