Authoritize by Milhouse Ltd
Authoritize provides strong two-factor authorization for logging in to any Mac running OS X 10.5 and above. Although still in development a beta version is available from the home page.
Authoritize stores the AES keys in the OS X Keychain, a Preferences Pane is used to associate Yubikeys with users and to view stats such as last login and key usage. On the drawing board for future versions are the option to use a server either local or remote, such as the Yubico server, to authenticate the keys.