Zeus preloads your app to let you start rake tasks, servers, console, generators, and tests in about 300ms.
It reloads itself (faster than you'd think it should be able to) when any files change.
Screencast (it's even faster now): https://vimeo.com/burkelibbey/zeus
Try it, and let me know how it goes! (seriously, contact me at firstname.lastname@example.org, @burkelibbey on twitter, or open a github issue on the project)