From Donald E. Knuth's MMIX page:

MMIX is a RISC computer designed by the author to illustrate
machine-level aspects of programming. In the next editions of his
books The Art of Computer Programming, MMIX will replace the
1960s-style machine MIX.

More details can be found in his book:

  MMIXware: A RISC Computer for the Third Millennium, by Donald E. Knuth
  (Heidelberg: Springer-Verlag, 1999), viii+550pp. ISBN 3-540-66938-8.
  (Lecture Notes in Computer Science, no. 1750.)

Some MIX programs have been converted to MMIX, and can be found at
http://www.mmixmasters.org/~mmixmasters/