Turbocharge Your Ruby Testing with Parallel Specs
In Make Your Test Suite UNCOMFORTABLY FAST! (called "the best blog post ever written" by one commenter) Jason Morrison of Thoughtbot demonstrates how to use Michael Grosser's Parallel Specs project to speed up your Ruby tests.
Parallel Specs provides a set of Rake tasks to run specs and tests in parallel, therefore using multiple CPUs (or cores) to multiply your testing power. It does not yet work with Cucumber features but Jason recommends testjour for that purpose - which is designed to work across multiple machines so isn't quite the same thing.
Thoughtbot has found Parallel Specs typically provides a 30-40% speedup for their projects out of the box, so if you're doing a lot of testing (and the best developers seem to say you should be) check it out for some instant satisfaction.