Great blog featuring a series of tutorials on complexity in computation, the theoretical limits of the Turing Machine, and other bits of required knowledge for the 21st Century. I wish that'd we'd studied this stuff in some of the math classes I took through high-school and college.
In Lesson 1 we described the Turing machine model to answer the question, "What is a computer?" The next question is "What can we compute?"
(via Kottke)