I've decided to set the bar high by looking for sites that do over half-a-million uniques per month or belong to a truly substantial company. A good rule of thumb is "do they have a Wikipedia entry?"

Here's what I've found so far: 

• Angelica (Publicly-traded health services company)
• Avis (The car rental company in Nicaragua)
• Danone (A lot more than just yogurts)
• Derbi (Motorcycle maker - part of Piaggio)
• English National Ballet (England's flagship ballet company)
• IBS Software (Software for airlines)
• IT Wire (Most popular IT site in Australia)
• Jetstar (Airline owned by Quantas)
• Latvia (The country's official site)
• Mistubishi Venezuela and Poland (The car manufacturer)
• Mountain West Sports (Cable TV sports channel owned by CBS)
• Nikon Instruments (High-quality technical products)
• Olympus (Camera maker's Australian site)
• Peg Perego (Italian child accessories)
• Porsche (Brazilian site)
• Quizilla (Heavilly modded Joomla site owned by MTV)
• Scandinavian Airlines (Official crew guide)
• Schneider Electric (World leader in power and control solutions)
• Sprint (Customer feedback site for cell-phone company)
• TNA Wrestling (Pro-wrestling organization)
• UNRIC (Branch of the United Nations)
• Vodafone (Cell-phone company's Icelandic site)
• Visat (Danish site for Scandanavian TV and telecommunications company)
• Yamaha (Musical instrument company's Mexican site)