Finally! Box.net has always had the ability to preview files, but it was through their Open Box system so 1. it sent the file to a 3rd party and 2. it was too slow.
This new file viewer works right in Box.net and is quite fast.
I can’t count how many times I downloaded files from Box.net just to see what it was, only to have to dump them from my Downloads folder later. This is a much better solution.