16 bits per channel
Yesterday, Krita reached a major milestone. We can now load, manipulate and save rgba images with 16 bits to the channel. This is really, really great. The only free software gui application that can do this too (that I know of) is cinepaint. ImageMagick can load, save & manipulate (and in fact, we still use ImageMagick to load and save the images), but it isn't a gui application.
Of course, there are limitations, currently. Painting works, and you can invert images, select bits and scale the image. The filters, histogram and color adjustment tools don't work yet. But that's only a matter of time.
RAW and openEXR, here we come!