Rutgers University, with more than 50,000 students the largest institution for higher education in the state of New Jersey, switched their main website, http://rutgers.edu, to Drupal. Looks stunning!
Mattel, the world's largest toy company, just relaunched Mattel.com on Drupal. The site was built by Vancouver-based work [at] play. Mattel products include Barbie dolls, Hot Wheels, Matchbox cars, Masters of the Universe, and much more. For once, my kids and family will understand what I'm talking about.
With the help of Cognisync, Sun Microsystems converted Java.net, the website of the Java community, to Drupal. The previous version of java.net, was custom built by O'Reilly Media. Interesting choice for a site devoted to Java, but needless to say, a great testimonial to Drupal.
Internet Systems Consortium, also known as ISC, is using Drupal on their website at http://isc.org. ISC was founded by three internet pioneers Rick Adams, Carl Malamud and Paul Vixie to support BIND and other software that helps power the internet.