A system of personality classification developed by Kathleen Briggs and Isabel Myers way back in the 20th century. Claims to be based on Carl Jung's ideas in his book Psychological Types, but radically revised by David Keirsey to be more in line with a great many other theories about personality going all the way back to ancient Greece.
The Myers-Briggs Type Indicator (MBTI) is a test, devised by Briggs and Myers, which you have to pay to take. Don't call it a "test" around people who give the test, though. They'll insist that it's an "indicator".
The Keirsey Temperament Sorter (KTS) is a different test, based on Keirsey's redefinitions of all the terms, which is available on-line for free (see below).
There is a second level Myers Briggs Two
Consult the Myers Briggs Types, for reference.
See Myers Briggs Discussion for a discussion of the validity and/or usefulness of the Myers Briggs personality test.
See My Myers Briggs Type Is... for quick statements of your test results.
What it is:
Listing of types:
"Temperaments" (groups of types):
Take the test:
www.keirsey.com 
- "The Keirsey Temperament Sorter II" (...with slightly different questions than those in the book.)
keirsey.com - "The Keirsey Character Sorter" (more ranking things, and fewer yes/no questions)
www.ling.rochester.edu . - "Duniho and Duniho Life Pattern Indicator"
www.personalitypage.com - $5 for a slightly better test.
[Note:
Competing Myers Briggs tests were forced offline by Keirsey. -- Jeff Grigg]
Distribution of personality types:
keirsey.com - of people taking the Keirsey test on the web.
Other personality measures:
Nine Personality Types / Ennea Gram [a "new age" thing]
Does anyone understand the ties between MBTI and Jungian archetypes?
Theologians go to www.mmcis.com . -- Jim Coplien
People with the "NF" Myers Briggs combination are called catalysts. They're big on relationships and teambuilding.
Beyond personality:
greenlightwiki.com A wiki about Lenore Thomson's more Jungian approach, in which the four-letter codes are seen as names for attitudes that people take as well as ways in which they have limited themselves.
www.typetango.com A dating site based on the Myers Briggs personality theory.
I would suggest consulting Steve Myers (no relation) and his viewpoint on contribution by type to the team:
and
-- Dave Fischer
See also Lipson Shiu
Book: Managing psychological factors in information systems work
Moved very insightful and elaborate summary to Myers Briggs For Programmers.
See original on c2.com