There is an MBTI test on facebook that isn't especially well designed in my opinion, but it's not terrible. It seems to mistype about as frequently as the standard MBTI.
http://apps.facebook.com/my-type/
However, it has an interesting companion function that allows people to take a test to type their friends. Obviously, self opinion and the opinion of others is likely to vary, but I find the possibility of others taking a quiz to show their opinion of you interesting, especially for those who have ambiguous self opinions.
So far, most of my friends' results type me as ENFJ, with a few typing me as INFJ. Not unsurprisingly, my external perception is more extroverted than my internal perception.
This approach could become an interesting tool if designed properly.