I use GIMP which is a free download, there is a fake HDR effect plugin for that. Not sure if its any good, I've only played with one image so far and it looked garish to say the least, maybe I chose the wrong image
Plenty of setting options with it though, so it might work out OK, it is only an effect though, not true HDR software
There is also something called Qtfsgui which is again a free download at
http://qtpfsgui.sourceforge.net
This claims to be a true HDR software and is a stand alone installation which doesn't require any other software to work