Since there's bound to be some discussion about MBTI's validity now that you brought it up, I'd like to point out that there's some interesting research emerging that seems to reliably correlate MBTI type with observed brain activity patterns:

http://www.youtube.com/watch?v=MGfhQTbcqmA

http://www.slideshare.net/AnnholmMSCCC/identifying-neurocorr...

http://www.keys2cognition.com/papers/EEGandSocialCognition.p...