Adobe Photoshop CS+;
Adobe Dreamweaver 8+ (or any php-editor);
For uncompressing a template ZIP package:
WinZip 9+ (Windows);
Stuffit Expander 10+ (Mac);
You can also check the Drupal repository for free themes at:
Many of these themes can also be previewed at the Theme Garden.
Please bear in mind that the contributed themes at those two sites are not part of any official release and may not work correctly.