Who are you, anyhow?
The Myers-Briggs Type Indicator is a popular personality test that can help you understand yourself and other people better. It’s based on Carl Jung’s ideas about introverts and extroverts, and the four functions of thinking, sensing, feeling, and intuiting.
Free online test
Keirsey Temperament Sorter
Keirsey is famous for publishing Please Understand Me, a book all about type theory and understanding your type. He offers some good resources on his website, including a free test and descriptions of all 16 types.
I’m a big fan of the folks at Personality Junkie for their interesting analysis of some of the types, and what might be good career choices and relationships for them. They also have some info especially for the rare NF types.
The Myers & Briggs Foundation
This organization carries on the work of the original creators of the MBTI. They offer a (paid) online type assessment and resources for professionals and the public.
Empirically-Based Revised Type Theory
James Reynierse has published an article discussing a revisied MBTI type theory that treats type as linear preference scales, rather than through type dynamics.
I hope these resources help you figure out more about yourself and your preferences!