Caucho
Creators of the Resin Application Server and the Hessian Binary Web Service Protocol.
http://www.caucho.com

HessianCPP
Aldratech's C++ client library of Hessian.
http://hessiancpp.sourceforge.net/

HessianPHP
Manolo Gómez PHP client and server implementation of the Hessian protocol.
http://hessianphp.sourceforge.net/