Here you can see four collections of Free WordPress themes .  More than 100 Free WordPress themes in four collections, but on the first collection check the  "related post from the past" section to see more collections on that website.

1. Weblogtools collection - 12 WordPress Theme

Check the related post from the past section there you can see more than 100 WordPress themes

2. 50 Free WordPress Themes collection from  RizwanAshraf

3. 32 Free WordPress 3.0 Themes from WP Robot

4. From Blogging Inside

This is not a collection link, So you will have click the "Older Entries »" at the bottom of the page to see more and more.