Spin preloads your Rails environment to speed up your autotest(ish) workflow. By preloading your Rails environment for testing you don't load the same code over and over and over... Spin works best for an autotest(ish) workflow.
Parallel and Distributed Test Runner for Ruby
Distributed test running with autodiscovery via Bonjour (for Cucumber first)
RSpec extensions for parallel, distributed execution across hosts, using DRb and Rinda