Google closure compiler

Google recently release Closure Compiler service (labs version)

use for optimising a few simple functions in the service’s web UI for JavaScript.

You can try to execute JavaScript for optimisation, check for warning and error, compressed into small files, see what is the post data looks like and more. Of course, this sort of thing can be easily done with Firebug but FireBug still lacks some features such as JavaScript file compress and a feature of “how not to program this way”.
It is worth to check it out.