With his post nominating apps for the upcoming Apple Design Awards (which we mentioned here), Scott Stevenson brought my attention to Locomotive from Ryan Rauum. Locomotive is an environment for developing Ruby on Rails applications on your Mac. It allows you to "try Ruby on Rails without worrying about breaking your current system - it's entirely self-contained." You just download it and "jump right into Rails development the minute you finish downloading." This seems a lot like MAMP, but for Ruby on Rails. So if you've ever wanted to try out Ruby on Rails (which is apparently how all the cool people are developing for Web 2.0) without the annoyances of getting Ruby and Rails properly set up, this is a perfect opportunity.
Locomotive is a free download from SourceForge, but donations are requested.