HDRtist (Free)
HDRtist on the web

This is a really simple app for Mac users to create HDR (High Dynamic Range) photos. All you have to do is drag and drop one or more photos onto the app window and it will create an HDR photo.

It obviously works best when combining more than one exposure. Here you can see the results of 2 exposures that I combined:

Pretty much the only thing you can adjust is the strength of the effect, but obviously you could edit the resultant HDR in another image editor to do any further tweaking.

Verdict 4/5 - Surprisingly effective for a free app and the ease of use is a real bonus. Obviously lacks the sophistication and fine tuning of specialist software like Photomatix Pro, but then it is £65 cheaper and is well worth a download if you want to create HDR pics with the minimum of effort.