He has been my hero ever since I read the first few pages of his outstanding work "The Art Of Computer Programming". That was when I was 21 years old, and although I understood only 20% of that book, I was very impressed by his writing style, depth and humor (the only other author whose technical books I love for the same qualities is Andrew Tanenbaum).
Nowadays Don seldom gives a public appearance, so I was pleasantly shocked when I saw him ask questions in a talk given by the creator of xkcd comics (see this strip to understand his question).
More about him on his birthday :
Coding Horror Blog
Recursed Blog