GIMP (GNU Image Manipulation Program) is actually a pretty powerful Image Manipulation Program developed by the open source community. I won't say its as powerful as photoshop, but it has many of the features that Photoshop has. I use it often, and find that it meets most of my needs.
HDR, by strictest definition, can't be done in GIMP since it only supports 8-bits currently. There is a Script-fu that can blend several exposures to create an enhanced ranged result (not HDR, but, for 8-bits, is quite useful). Also faking Tone mapping is quite easy in GIMP (script-fus for this too). Don't have the links anymore, but I got them at the Registry.
