<script type="text/javascript" language="javascript">
            function canc()
            {
                document.getElementById('<%=txtContattaci.ClientID %>').value="";
            }
        </script>
        
        <asp:TextBox ID="txtContattaci" runat="server" Columns="50" Rows="15" TextMode="MultiLine" />
 <input type="button" value="Svuota" onclick="canc()" />