I came across this video (sorry about the ad... first time I've seen ads on youtube videos) by Patrick Norton and Roger Chang of Revision3 the other day that besides being a decent basic introduction to HDR talks about an open source HDR processing application called Qtpfsgui. This is the first time I'd heard of the application. It runs on Windows, Mac OS-X (Intel only) and Linux and I have to say that, after downloading it and installing it on OS-X, it works pretty good.
I tested it by processing a 7 exposure HDR series that I'd processed with Photomatix. I processed the same images with Qtpfsgui and with all the default settings the final image came out as good as what I'd done with Photomatix. It took me a couple of tries to produce a finished image mostly because it's a little different than Photomatix. I'm amazed by how well it works. There a a bunch of features that Qtpfsgui doesn't have, like blending, exposure fusion and a Lightroom plugin so I'm not going to say Qtpfsgui is as good as Photomatix, but For those starting out in real estate photography this could easily be a way to save some money. Qtpfsgui is donationware and If you use it on a regular basis I highly recommend that you make a donation to the authors.
I missed a step in the installation that took me a while to figure out. You have to download QT4 and install it before Qtpfsgui will work. Give it a try and see what you think.