What are some alternatives?
When comparing YubiKey and SoloKeys, you can also consider the following products
Authy - Best rated Two-Factor Authentication smartphone app for consumers, simplest 2fa Rest API for developers and a strong authentication platform for the enterprise.
Google Titan - A 2FA device to protect yourself from hackers
andOTP - andOTP is a two-factor authentication App for Android 4.4+
Nitrokey - Email Encryption. Encrypt your emails with GnuPG, OpenPGP, S/MIME or your favourite email client.
Google Authenticator - Google Authenticator is a multifactor app for mobile devices.
YubiKey NEO - Hardware-based authentication