Nitro has been around for a while: providing products in a similar space to yubico. Not particularly competitive on price or form-factor but I gather the "Made in Germany" tag has made them popular in Europe.
Nitrokeys: Smartcards + MCU and rather shitty GUI software for controlling them, compatible with standard crypto tools like gpg-card. Gen < 3 is missing the hardware button that Yubi has. The HSM model has a fancy smartcard. The Start is a triple GnuPGCard with usb.
The Pro can be used for attestation with HEADS firmware.
The Nitropad is just a refurbished x230 with coreboot and heads. Kinda pricey for my taste.
The open-source nature of a lot of their offerings is a big selling point though, you're right. I overlooked that