phpcow is too good to be true... but I called 12 company from their testimonial phpcow.com/?c=121 and they confirmed testimonials are real. looks like i'll get a monthly hosted today.
So many newbiews talking positive raises some flags in my mind. Anyways, not that I don;t like phpcow, but you know what I mean...!!
I believe that the CMS used by Indiemedia is open source, and Geeklog can handle such a job quite nicely as well, lots of Geeklog powered sites are 'Slashdot' proof.
I know most people here use php, but if you use java, some of the CMS's are: Liferay Magnoilia Cofax openCMS Alfresco
Some of the larger companies use the magazine.net software this company offers http://www.idcubed.com/products/magazine.net.aspx . You can view some of the projects they've done under their portfolio.
Many sites are using it! But because of the radical template abilities it's just really hard to tell. Oh and I thought it was funny by America's Army or whatever that free US army-made game was, used PHP-nuke... ewww. Site looked good, but the code was.... a living testament to how link popularity matters way more than whatever is in your code! That's for sure.