I just want to note that the new CoffeeScript compiler is using ESCodeGen as a backend, which is doing the source map heavy lifting. Anyone writing a compiler which targets JS can use ESCodeGen as a backend and get source maps pretty much for free. Awesome news all around!
Note: I do not want it to seem like I want to take anything away from Michael Ficarra and the new CoffeeScript compiler. He also wrote the code to integrate source maps with ESCodeGen, but it is something that many projects will be able to take advantage of, not just CoffeeScript.
I would not recommend you use it in production just yet, but from my reading of that wiki page, there are only 3 or 4 fairly uncommon features that are unsupported. Most of them could still use more tests, though, and that's why they are missing that last checkmark.
https://github.com/Constellation/escodegen
Note: I do not want it to seem like I want to take anything away from Michael Ficarra and the new CoffeeScript compiler. He also wrote the code to integrate source maps with ESCodeGen, but it is something that many projects will be able to take advantage of, not just CoffeeScript.
In other source map news, if anyone hasn't seen it yet, UglifyJS v2 now supports source maps: http://lisperator.net/blog/uglifyjs-v2-news/
Source map support is on the way for Firefox's debugger as well: https://bugzilla.mozilla.org/show_bug.cgi?id=771597