If I understand you correctly, you want something to display the links in two columns. Try this ...
echo "<table><tr>";
while($query_data = mysql_fetch_array($result)) {
echo "<td><a href='business_list.php?category=$query_data[id]'>";
echo $query_data[category];
echo "</a></td>";
$i++;
if ($i == "2") {
echo "</tr><tr>";
$i = 0;
}
}
echo "</tr><table>";
You will probably want to include some logic to check if you have an even number of links, as browsers differ in the way they handle table cells that don't exist.