Cinepaint
Cinepaint is an Open Source image editor developed for the movie industry specifically to support high dynamic range image editing. Although originally based on the gimp image editor (for a long time cinepaint was called Film Gimp), this application has since diverged significantly.
Note that the Panorama Gimp plug-in doesn't yet support HDR or 16bit images, so there is no advantage using cinepaint over the gimp with this plugin.
There is a thorough cinepaint tutorial on manipulating 16bit photos.
A useful cinepaint plugin is Hartmut Sbosny's Bracketed exposures to HDR plugin for combining bracketed exposures.