Yes, he is said to use Emacs. You can even find a library or two on his home page:
Notably there is this emacs lisp package:
It lets the user highlight lines with different colors, which are persistant across file-saves and re-loads (a line in color “a” is saved with a leading Control-A, the saved file begins with the line “Color-Mode”, which kicks you back into color-mode when it’s re-loaded).
- “Some parts of me like mathematics, and some parts of me like emacs hacking.” – All Questions Answered (pdf)
- “I use the Emacs editor very heavily”
- “So now I don’t read electronic mail, but I do use it occasionally. Say I’m taking a trip to Israel and I’ve got to make last minute arrangements. When I visit another university or research center for a few days, I have to send email from there. I’ve learned how to use the email facilities in Emacs, but I don’t want to get good at it.” – http://tex.loria.fr/litte/knuth-interview