The best code I've seen: -
Common Lisp - "Paradigms of Artificial Intelligence Programming" by Peter Norvig and "On Lisp" by Paul Graham
C - "C Interfaces and Implementations" and "LCC- a compiler for ANSI C" both by David Hanson. I also found the code for the Player/Stage robotic sim framework surprisingly readable.
I liked the Scala Actors library code as well.
If any HNers know any great codebases in Haskell or Erlang, please post here.