transpose 2-dimensional Javascript array in one line

There are quite a few libraries that implement a transpose function for arrays, but sometimes you don't want the hassle of installing them or the added dependencies. I'm sure I'm not the first to use this approach but most snippets found by a quick search are too verbose to comfortably drop in for a single use.
Update April 2014: It occurred to me the other day that the magic mentioned below is unneeded, as map already provides the column indexes as the second argument. I've updated the gist so the following remark is no longer relevant. The most magical bit here is to use Object.keys() on the first row of the array which yields an array of column indexes, which we can then use map() on to extract each column in turn. Love to see a terser implementation if you've found one.