The Thickbox module is a wrapper for the jQuery plugin ThickBox.
Drupal 7 and Colorbox as the recommended upgrade path
I will not release a Drupal 7 version of the Thickbox module. The Thickbox script is getting a bit old and is cumbersome to maintain well for all new browsers.
Instead I recommend the Colorbox module that I'm a co-maintainer of. I have ported over all of Thickbox features to it and then some, I think you will like it. Please try it out on your Drupal 6 test sites.
You find the Colorbox module at http://drupal.org/project/colorbox.
Report your experience here http://drupal.org/node/750332.
Please observe that you need jQuery Update version 6.x-2.x to run Colorbox in Drupal 6.
From the ThickBox homepage:
Thickbox 5.1.x is compatible with Imagecache 1.x.
Thickbox 5.2.x is compatible with Imagecache 2.x.
Comparison with Similar Modules
Please read the article at http://drupal.org/node/266126