I’d like to suggest SoloKeys for the MFA page as another option under the “Hardware Security Keys” section.
SoloKey devices use open source software (on GitHub) and hardware, though I don’t know too much about the hardware part or how it compares to Nitrokey.
Some background (so others don’t have to start research from scratch)
Upcoming SoloKeys devices in the Solo V2/Solo 2 line (unless you already knew about SoloKeys) seem to be most similar to the Nitrokey 3 line. Currently, the Nitrokey 3 products are in pre-order status (for the USB-A and USB-C variants).
As stated in a SoloKeys announcement from February 2021, SoloKeys and Nitrokey are competitor-collaborators because both companies use Trussed, the same open source cryptography framework.
SoloKeys doesn’t seem to get a lot of attention, because SoloKeys only makes hardware security keys (so far). On the other hand, Nitrokey is more well known because it sells other products, such as the Qubes Certified NitroPad X230 and T430 and the NitroPhone 1 and 2/2 Pro (which are preinstalled GrapheneOS devices on the Pixel 4a and Pixel 6/6 Pro, respectively, with various options for removing the microphones, sensors, and cameras).
Other consideration: shipping availability
I originally wanted the Nitrokey FIDO2 but accidentally bought the Nitrokey Pro 2 (which can be used for unlocking the computer upon boot, like in the Insurgo PrivacyBeast X230) in late summer 2021. However, long story short I remembered in a video about passwordless account logins (it’s a bit idealistic) that SoloKeys is a good alternative to Nitrokey devices (since shipping was €50 or more via only UPS due to German COVID mail restrictions in early fall 2021).
Basically, if you’re in the U.S. or close to North America, then SoloKeys is more sensible regarding shipping - while those in the EU should consider Nitrokey for similar reasoning. However, both SoloKeys and Nitrokey will ultimately ship internationally. Having 1 more recommendation for hardware-based MFA alongside YubiKey and Nitrokey could help readers regarding availability.
I’m wondering if there’s any progress on suggesting SoloKeys since then.