Code Generator is a time based code generation app for not just facebook. It creates a unique login code every 30 seconds, even when you aren't connected to the internet, so you always have a code when you need one. If you have login approvals turned on, you can use Code Generator instead of waiting for a text message with your login code.

How to use for facebook:
Open your facebook account on your personal computer and go to settings->security setup code generator.