I wrote a tool along those lines: http://github.com/mwylde/slinky.

It runs a static file server that transparently compiles resources as they are requested (so you request main.js and it finds main.coffee, compiles it, and sends it back).

Then when you're ready to deploy, it's one command to compile/concatenate/minify your css and javascript.

Right now it only supports Coffescript, SASS and HAML, but adding additional compilers is really simple.