mobileOTP is what you are looking for:

http://motp.sourceforge.net/

it supports a number of OTP flavours (OATH-HOTP, OATH-TOTP, mOTP etc.). GOOG and others also provide OATH apps, ie:

http://f-droid.org/repository/browse/?fdfilter=OTP

all FLOSS and work with standard services. If you prefer a hardware token, inexpensive Yubikey tools come with every Linux distribution.