Ethereum currently has a working proof-of-stake network. As far as I know it's the most scalable version currently live, capable of millions of nodes without any sort of delegation. Right now it has over 100,000 nodes and has run perfectly since it launched on Dec. 1.

However, so far it doesn't have users other than the stakers themselves. It's not all that hard to transition everything over, it's just that it's a live $200 billion economy and they're being really careful about it. Should happen next year though.