Hal was co-lecturer of 6.001 when I took it in 1983. I've already blogged about how he made me reconsider my opinion of Lisp by pre-empting my objections about efficiency and getting the machine to do calculus on day one.
I was also amazed by Hal's book Turtle Geometry. Hal was involved in
the development of Logo for the Apple II computer. I had heard of
Logo as a `kid's language', but I discovered that it was a bit more
interesting than that when I flipped through Hal's book. The first
chapter involved the pretty pictures you could generate through simple
iterative and recursive programs, but the later chapters involve
vector analysis, topology, linearity, symmetry groups, invariance,
and differential geometry. The final chapter introduces General
Hal has a way of taking very complex ideas, extracting the core
concepts, and reframing them in an intuitive and easy to understand
manner. I'm excited that Hal is doing a sabbatical at Google and I
hope I can help him out.