I'm a huge fan of the Myers-Briggs personality tests.
Here's a simple online unofficial version: http://www.humanmetrics.com/cgi-win/JTypes2.asp
What does it measure?
I realize there's criticism of it, but in my personal experience, it's done a good job of partitioning the problem space regarding people's personalities.
Here are the 4 dimensions in the test, plus my simplified description of what each means:
* Introvert (I) vs. Extrovert (E): How energized are you be other people? I finds being in a crowd draining, whereas E finds it energizing.
* Sensing (S) vs. Intuition (N): N means more likely to go with a gut-feeling. S is more by-the-books.
* Thinking (T) vs. Feeling (F): How much do you emotions weigh into your decision making?
* Judging (J) vs. Perceiving (P): J means more punctual and driving for closure. P is more flexible.
I'm an INTJ. Anybody who works with me should be able to guess that. I show up as INTJ every time I take the test, so that seems to be pretty stable. I tried the above link and got INTJ there too, so while it's not an official test, it had sufficient accuracy for me.
Why it's valuable in a work environment?
Here are my personal opinions on why it's valuable, particularly in a work environment. As these are just my personal opinions, you'll probably disagree with them.
1. No personality is overall better than an other one. However, each personality has strengths and weaknesses. When you're on a team, you need to know your teammates strengths and weaknesses so that you can leverage their strengths and avoid their weaknesses. This is also essential for setting people up for success.
2. It's good for predicting where people will have tension. For example, A strong-N and a strong-S will have an extra burden communicating because they tend to operate at different abstraction levels and so can easily talk right past each other. I've also personally seen a lot of tension between strong-Js and strong-Ps.
(Other trivia: If you spell "Myers" wrong in your search, you'll find Firaxis Games because of Jeff Briggs and Sid Meiers.)