YubiKey
A popular security device used, among others, to secure access to various Bitcoin services.
For example, it is used on Mt. Gox and My Wallet.
See also: - http://security.wikia.com/wiki/Yubikey - http://wiki.yubico.com/wiki/index.php/Yubikey
Security model
It is important to understand that Yubikeys use Symmetric Encryption (specifically, AES).