A quick implementation of of "HOTP: HMAC-Based One-Time Password Algorithm", RFC 4226. Specifically designed to produce authentication codes for Google two-factor authentication.
When setting up two-factor authentication with Google, first scan the QR code with a generic QR reader app. Type the URL decoded from your QR code into the file ~/.google. You can put one URI per line. Then simply run the main class net.drwille.otpauth.OTPAuth.
Just use Maven to do a mvn package. This will create the jar.