The so-called reference implementation is the version of AMP shipped with Twisted.
Documentation and code here:
http://twistedmatrix.com/trac/browser/trunk/twisted/protocols/amp.py
Example server: http://twistedmatrix.com/trac/browser/trunk/doc/core/examples/ampserver.py
Example client: http://twistedmatrix.com/trac/browser/trunk/doc/core/examples/ampclient.py