He has enormous piles of data about himself and his activities,
including every email he's ever sent or received, all the keystrokes
he's made, every calendar event, detailed logs of telephone call start
time and duration, and fascinating data about his editing of his book A
New Kind of Science. He's also apparently scanned and OCR'ed almost all
the paper documents he's ever received.
What's interesting to me is that he seems to never have looked at any of
this data until now! It's a fascinating look into his life. I'd love
to have someone do that email analysis on my archive, but I just know
that it's not as easy as he makes it sound - my archive is very
heterogeneous, since I change software about every five years...