Don't forget StackOverflow: http://highscalability.com/blog/2009/8/5/stack-overflow-arch... - apart from the somewhat unbelievable PlentyOffish project, it's the only prestige project based on MS software most people will recall. In their 2011 update, they claimed to be using "a lot more Linux machines", because apparently it's fun to develop using Microsoft technology, but for some infrastructure components (proxying, Nagios...), there's no way to beat Linux even if you're a MS fanboy. ;-)
Plenty Of Fish shouldn't be considered unbelievable. I've been following information about their infrastructure from the first moment they posted about it. There's nothing unbelievable about it at all. Over the years he has openly described changes to the site's setup as they've scaled, and it makes fine sense.
Stack isn't the only prestige product. Nearly all of Microsoft's infrastructure runs on their own dog food. That's a lot bigger of a product than Stack Exchange is.
