Simplest way i can think of:
$Selectid = "SELECT website FROM banners WHERE Username = '$Username' ";
$Result = mysql_query($Selectid);
if (mysql_num_rows($Result) >0)
{
// creates array for each site url
$users_row = mysql_fetch_array($Result))
then put <?php print "$users_row["sitefieldname"];" ?> wherever you need the site url put, just create the list menu in normal html.
I hope i understood what u wanted, if not just post and clarify what you need.