Speakeasy is a one-time passcode generator, ideal for use in two-factor authentication, that supports Google Authenticator and other two-factor devices.
195K downloads/week, MIT license
Time-based (TOTP) and HMAC-based (HOTP) One-Time Password library
It implements both HOTP - RFC 4226 and TOTP - RFC 6238
393K downloads / week, MIT license