DRb is Ruby's RMI system that allows Ruby processes to communicate with each other transparently. DRb is included in Ruby 1.8, for Ruby 1.6, you can download DRb from the RAA.
I've written an introduction to DRb that explains how to use DRb when connecting Ruby processes, and how to use DRbUndumped.
DRb uses id conversion to map ids into objects when referring to remote resources. This article explores DRb Id Conversion, gives examples of several methods of id conversion built-in to DRb and shows how to write your own id conversion class.
You can learn how to set up DRb over SSL which uses QuickCert to help you tunnel DRb through SSL.
I presented "Distributed Ruby an Introduction and Overview" at the Silicon Valley Ruby Conference. You can download a PDF of the slides or the Keynote original.
Rinda is a Ruby port of Linda written by Masatoshi SEKI and is included in Ruby 1.8. Rinda consists of a TupleSpace implementation and a RingServer that allows DRb services to automatically discover each other.