GearUp!
GearUp is a simple Gem that will start a thin server running sprockets (rails asset-pipeline).
What does this means for you?
You have "some-directory" with some assets:
$ ls <some-directory> javascripts/application.js.coffee stylesheets/application.css.less
Switch to that directory:
$ cd <some-directory>
Gem install 'gearup':
$ gem install gearup
GearUp!
$ gearup
Open "http://localhost:3000/javascripts/application.js" in your favourite browser and note the compilation
NOTES
Played with on JRuby 1.6.7.2, 1.9.2, 1.9.3.
Feedback
Let me know, if your using it and/or if you have any trouble.