Offhand I can think of a few: ThoughtWorks uses it on most of their projects afaik. Github I believe does as well. They published an interesting blog on their deployment process a couple of months ago.

http://scottchacon.com/2011/08/31/github-flow.html

Where I work at LivingSocial the web application is deployed daily, sometimes dozens of times a day. I've recently taken over the Android project's infrastructure and we're in the process of implementing Continuous Delivery for that app as well.

David Anderson gives examples of implementing it at both Microsoft and Corbis, Bill Gates' other software company.

Etsy has done some excellent work on Continuous Deployment systems, so I assume they are using this...

GitHub seems to fit that bill quite nicely.