Where can I download old releases of Drupal? I'm trying to find a copy of 6.20 to run a diff on a client's Drupal install. I believe there's been changes to core. 

How can I get version 6.20? Additionally, how can I download any older Drupal versions?
You can get Drupal 6.20 from the Drupal 6.20 released page.

You can also view a list of all current and previous releases on the Releases for Drupal core page, complete with download links. It goes all the way back to Drupal 4.2.0, which I'm now off to install to see what happens!