Looking for free-access software to develop high dynamic range (HDR) imaging, I found this tutorial for GIMP. I know it is a little bit long, but it is worth it to go through it.

http://www.instructables.com/id/SL3MV0OF2AX8OVC/

Nice technique. You can use this for any photo software that supports layers and layer masks, not only the gimp. There are some free pieces of software that can do more with less work. I'm familiar with qtpfsgui. It works with higher bit depths than the gimp, and is a bit easier to use because it allows you simply to change the look using sliders:

http://qtpfsgui.sourceforge.net/

You can download it for Windows, Mac, or Linux.