I may be a little late to the game here, but I recently discovered Zeus and since then my mind has been blown.
@LivingSocial I was always trying to find ways to make my tests faster and faster. Having such a large monolithic rails app it became increasingly slower and slower to boot your rails environment before running a test.
This resulted in me creating separate test groups that didn't necessarily need all of rails to be loaded in order to test.
Well now thanks to Zeus that isn't even necessary. Zeus basically boots your rails app for you, and then whenever you call commands through it, your environment is preloaded and it responds almost instantly.
Best of all it takes seconds to setup on any rails app.
After completing that I saw immediate results. Upon running the offers unit tests it went from taking 6.628s to 2.050s. An instant win that took literally a minute to setup.