PureBoot is the name we use for the suite of software we use to secure the boot process. Practically speaking this ends up usually just referring to our fork of Heads, but technically speaking also encompasses other parts of securing the boot process, including a script to allow you to unlock your disk using a Librem Key. Those parts are less mature than the boot firmware, though, so normally when we talk about PureBoot we are talking about our custom fork of Heads.
I’ll elaborate a bit more on this. Like @MrChromebox says, only the Nitrokey and Librem Key are supported, but when we talk about “supported” here we are specifically talking about the HOTP tamper detection function that we added to Heads when we first started offering it on our laptops.
Before we added this feature, Heads would use TOTP only, which means you would take a picture of a QR code that represented the shared secret stored in the TPM, and use a phone authenticator app to convert it to a 6-digit code. Each time you boot you would take out your phone and verify the code. I wanted Heads to be more user friendly because I wanted wider adoption among our own customers, and felt that if a user has to unlock their phone each time they booted, and compare a 6-digit code, they’d likely just skip it.
With the Librem Key HOTP support, it’s much simpler. Just insert the key at boot, and when the TPM releases a shared secret, PureBoot converts it to a 6-digit HOTP code and sends it over USB to the Librem Key. If it matches what the Librem Key generated itself with its own version of the shared secret, it blinks green, otherwise it blinks red.
You can use a Yubikey for GPG signing of files in PureBoot, but we worked with Nitrokey to write custom firmware for the Nitrokey and Librem Key to implement this special HOTP feature. In theory other USB security tokens could implement this HOTP feature, but no others have. At the moment, if you use a Yubikey to sign files in PureBoot, you have to fall back to TOTP to verify that the boot firmware hasn’t been tampered with.