Downloadable Drupal Distributions

Open Atrium “an intranet in a box: blog, wiki, calendar, to do list, shoutbox, and dashboard” by DevSeed
Acquia Drupal by Acquia
ProsePoint “newspaper content management system”, by Thinkleft
Pressflow “Drupal with integrated performance, scalability, availability, and testing enhancements” by 4 Kitchens
Everything under "installation profiles" on Drupal.org

Most of them technically Drupal install profiles.

SaaS Drupal Distributions