At the time of writing, the release of Drupal 5.x was only briefly past. Developers of the various templating engines were still working to port their applications to Drupal. While the 4.x series sports a number of templating engine options, including the popular Smarty engine and XTemplate, Drupal 5.x users were left with only one alternative to the default theme engine. Engines that are compatible with the 4.x series are not compatible for the 5.x series.
While at this time only one alternative is certified for Drupal 5.x, for purposes of our discussion here, I'll touch on the most popular alternatives to PHPTemplate. Odds are 5.x users won't have to wait for long before the developers of the popular systems below catch up.