While checking out Drupal for it's latest beta version (4.7 beta3 I came across a couple of REALLY COOL sites that were built on Drupal (also older versions like 4.6 and 4.5 of course…)

• Rental Monster – a real-estate site, that integrates Google maps and Cragslist, as well as direct subscription content … very cool!

• Trash Your TV – a complete guide to a TVfree lifestyle. Find out if you are addicted to TV, why TV is harmful, and what you can do about it. :) funny stuff

• BuyBlue – a community for rating ethical companies – with a huge data schema including sets of 1:n or even n:m relations for the company profiles like here or here —- make sure you click those detail links like the "Corporate and Social Responsibility" in a company profile post

• SpreadFirefox – I'm sure you saw this site before – it's a drupal one
• Cialog – Very distinctive look, great use of corporate design for a completely different, slick appearance
• eCademy – Widely known, with thousands of regular users. It might not fit the corporate billing completely, but it's still a well respected (and long running) drupal site.
• Sudden Thoughts – Innovative music review / information site. Just the unique front page alone sets it apart from the rest.
• Swedish Library site – of Karlstad – describes the services the library offers the students and the public. Also a database with there resources such as magazines. Uses some special tweaks to get multi lingual support.
• Terminus1525 – this is a REALLY cool community based art site built on Drupal (and tons of interesting uses for Flash) – for people sharing their photo artwork and blogs —- OMG - just checkout their SUPER-COOL picture browser – very very stylish!
• Thomas Eagle photography – a photograph's portfolio site – damn slow here – not sure why…
• Ill Recordings – a recording companies site with artis portfolios
• A bio-tech company website
• Hipfoto a turkish foto community
• A simple, straightforward IT company page
• PRonline.ru – I don't understand a word on this russian site – but it appears to be a public relations portal
• DonatGroup —- an interesting company site with a huge portfolio/reference area
• "Project Managment (PM) Days site":http://pmdays.pl/pl

Some government / municipal use

• Alpine County,CA
• City of Nogales,AZ
• Town of greenport

One interesting book on this topic might be this one… of course – reading thru 500 pages vs. trying stuff yourself is always a hard decision as we are all very time-constrained :-)

A couple of the sample sites came from the very interesting thread here – so you may want to check that out for more or updated sources :-)