An example is stack overflow. They've been able to scale up instead of scaling out.

http://highscalability.com/blog/2009/8/5/stack-overflow-arch... is the link I can find right now.

I'm not sure why you said "a single database server". No one said that. We're talking about sharding across databases, and the vast majority of sites don't need that.

Having a master/slave setup is more than a single server, but it's NOT the same as sharding.

On top of my head I can think of two: Wikipedia and leboncoin.fr (one of the largest websites in france).

EDIT: Both run read-only slaves. I was only talking about no sharding.