Where AMP was invented. Used as a reference for all other implementations.
Code and documentation here:
Eric also wrote a C# AMP client/server library. See AmpSharp.
Ben Doran has written an ActionScript 3 client/server lib here:
7-Eleven Stores has sponsored a Java implementation available on github:
Tristan Seligmann has written a Clojure implementation available here:
C library - libamp
Eric Mangold and Peter Le Bek have written a C library implementation of AMP. (Also usable from C++, or other languages that may easily bind to C-ABI libraries.)
See the libamp page for further details or to contribute.