Cappuccino does this. Compare the speeds of the work-in-progress (https://wip.laser-tags.net) and the same site (https://laser-tags.net) after doing a ‘jake release’ which pulls the 90 or so files into a single .sj one.

There’s also ‘jake press’, which goes through the JavaScript code, pulling out anything not actually referenced and binning it

(edit: and it occurred to me while in the shower, if anyone wanted to tell me where (roughly) you are located, and loading time for the 'released' site, it'd make me happy :)