One of the most commonly used tests to determine your personality type is called the Myers-Briggs Type Indicator (MBTI).
The test checks for four different personality preferences or dichotomies, where you can be scored at anywhere between the two extremes of the dichotomy:(I)ntroversion or (E)xtroversion
(S)ensing or i(N)tuition
(T)hinking or (F)eeling
(P)erceiving or (J)udging
This is the Wikipedia entry about MBTI.
You'll find the following links to external sources about MBTI in this part of the Wikipedia entry.
Much of the underlying theory behind personality types came from the work of Carl Jung and his book Psychological Types.
I've taken the MBTI test twice: once in 1989 and again in 2000. I don't have a copy of the 1989 score, but I was classified as an introvert at that time. I was classified as type ISTJ in 2000.
Anyone else out there want to share their MBTI classications?