We have found the Drupal Theme Garden very useful in the past. Since the Garden is closed now, we decided to provide new space for showing various themes in all its glory. Initiative to do that resulted in this site.
To begin with, we tried to make site similar to original one. However, we certainly plan to introduce an improvements in the future.
Themes you can try here are made for Drupal 6 and can be downloaded from the themes page on drupal.org.
All themes are shown as is.