Here is how I do it.

Create a page called random.php. Add the following code to it.

Code:
<?

  // Random HTML Generator
  // HTML Array....
$aHTML[0] = 'Iframe CODE 1';
$aHTML[1] = 'Iframe CODE 2'; 

  // Calculate index
  $iUpper = sizeof($aHTML) - 1;
  $iLower = 0;

  $iIndex = rand($iLower, $iUpper);
  // Display HTML
  echo $aHTML[$iIndex];
?>
Change Ifame Code 1 and Iframe Code 2 to the iframe code for the mod that you want to display. You can also enter any other code such as adsense ads. You can add additional lines by adding $aHTML[3] = 'Iframe CODE 1';. See that I changed the number in the [] to 3. Just keep incrementing that number for each new line.

Now that you have the random.php file working. You can use an iframe to add it to your page. It works really well.

Hope that helps, Jereme