This includes a plugin for spork to enable Test::Unit support for spork. It also comes with a very bare-bones drb client to run tests.
To use it, install the gem, then fire up spork in your project directory (the file test/test_helper.rb must exist to detect that Test::Unit is being used).
Then, once spork is running, invoke testdrb (e.g. testdrb -Itest test/your_test.rb) to run your tests under Spork. To use a port other than the default simply add -p 1234 or --port 1234 to you command (where 1234 is the port number that Spork is running on).