GnuStep
   An implementation of distributed objects written for GNU Objective C. It is similar to NeXTstep's Distributed Objects system.
   http://www.gnustep.org/

Scarab: Lightweight Distributed Objects
   A lightweight scheme for representing distributed objects providing both a compact binary protocol as well as XML-based serialization. Used as part of the Casbah project.
   http://casbah.org/Scarab/ldo.html