[weinzierl]

Nitrokey (formerly CryptoStick)

https://www.nitrokey.com

AFAIK they are used at Mozilla. The Firmware is Open Source. Downside is that not all their dongles support U2F.

[drdaeman]

Actually, none does: https://www.nitrokey.com/#comparison

The only dongle to support U2F is currently only available for pre-order, with ETA in autumn 2017.