Let people reset their password by answering secret questions.
What's your pet's name?
What street did you grow up on?
You can define your own question pool from the Dashboard.
You can even change your question pool or edit the phrasing of questions over time.
Once someone chooses the questions they want to use, their question and answer set is stored in an attribute against their user account so subsequent changes to the question pool don't impact answers already given.
This is a handy way to make it easier for communities where people may not have easy access to email to reset their password in the normal concrete5 fashion.
WARNING: Studies show that even people who don't know you can guess your answers as frequently at 17% of the time, so this isn't a great add-on for folks primarily concerned with security.