Fits liberator from NASA (free). Although
Eddie Trimachi does a nifty convertor with some other features. Check out his FIT4win2 also, a very usefull app indeed.
Most ppl use fits liberator, its the industry standard, because it allows simple 2 click stretching/scaling in 32 bit before conversion to 16bit tiff for PS .... and its free (NASA useses it for Hubble pics).