We use Duo for various things around here. Love it. It's much more feature complete than Authy (which, unfortunately will not work for any of our use cases).

We use Duo for various things around here. Love it. It's much more feature complete than Authy (which, unfortunately will not work for any of our use cases). More importantly, the company is run by well know and respected security researchers (Dug Song and Jon Oberheide).