Create the dropdown with autopostback
<asp:DropDownList ID="DropDownList1" runat="server" OnSelectedIndexChanged="Sel_Changed" AutoPostBack="True">
<asp:listitem >Google</asp:listitem>
<asp:listitem >Yahoo</asp:listitem>
</asp:DropDownList>
In the code
protected void Sel_changed(object sender, EventArgs e)
{
switch (DropDownList1.SelectedIndex)
{
case 1:
Server.Transfer("www.google.com");
break;
case 2:
Server.Transfer("www.Yahoo.com");
break;
}
}