Drupal
I've downloaded Drupal 6.10 for investigative purposes and have been impressed by the ease of installation.
After unpacking and configuring MAMP, I simply ran the install.php file and I was up and running in minutes, very straightforward. So far I've been digging around in the markup mostly, but have also taken a quick look at the user rights and privileges and also at a few of the themes to see how they're put together.
I also like the abstract way that the modules seem to be used and re-used around the site. I've only scratched the surface here, but am looking forward to looking into it further.