IPIP (International Personality Item Pool)
A public-domain library of personality test items researchers can use, translate, and republish for free.
The IPIP was started by psychologist Dr. Lewis Goldberg so researchers could study personality without paying licensing fees for commercial instruments. Every item in it is public domain.
The IPIP-NEO-120, developed by Dr. John A. Johnson (2014) from the IPIP, is the instrument behind our free Big Five test, it's why the test can be free, accurate, and fully cited rather than a paraphrased copy of a paid instrument.