So the other day I was asked what my Myers-Briggs type... where my questions was, "Huh?" It turns out I knew what it was, have taken the tests, and knew (with a 90% certainty) my type. I just didn't know what it was called (minor detail, that).
Anyway, I knew for a fact that there was an "I" in there somewhere, as well as a "J". I was mostly certain of the "T" part, and based on our conversation, we concluded that I must be an "N" as well.
I decided to look it up, and found a very good description of INTJ personality. Now mind you, since it is a generalization, it may not be 100% accurate for everyone, myself included. After reading it, I decided to take the test again and found a useful website, HumanMetrics, with a bunch of questions to answer. The result was INTJ.
There are people who state that their results aren't accurate at all, but the thing is, you have to be 100% brutally honest with yourself while taking the test. Throw out all of the things that parents, friends, lovers, acquaintances have told you of what you should be and dig deep down to the core personality you have and answer the questions.
I was actually surprised at how accurate the INTJ description was for me. I don't put a lot of stock in generalizations or tests of this nature, but I must be honest, this is pretty damn good. There are bits of the description that I'm sure most people would have never guessed about me based on our interactions. In fact, there are some things in the description where I had the thought of, "Huh, that does make more sense."
I'm not saying that you need to make everyone you know take the test, or that you need to know a person's personality type in order to interact with them successfully or develop any sort of meaningful relationship (whether it be friendship or romantic), but it sure does help. If for no other reason that to know yourself better, it helps.