The Radiance synthetic imaging system uses a picture file format (pic) that offers a floating-point representation of pixels. A gallery of Paul Debevec's high dynamic range images in "pic" format is here. Note that these files use a ".hdr" filename suffix, rather than ".pic".
vrpic is a simple GTK+ based viewer for images in this format. The viewing window has a slider control to change the image exposure; the units are f-stops.