There are dozens of tools out there dedicated to creating High Dynamic Range (HDR) photographs and among the most powerful, yet easy to use, must be the new Oloneo PhotoEngine, of which beta version is now available for a free download.
Dynamic range, simply put, is the difference between the darkest and lightest area in a photograph.
HDR, therefore, is the ability to show a greater balance between the two extremes in a picture, in a sense that there will be no areas in the photos which are too overexposed or underexposed (see the clip below).
However, HDR is not the only thing PhotoEngine is capable of creating. The tool can also be used to create a certain mood in a picture, as can be seen below. The first photo is the original.
By the way, you can download the HDR tool here. Have fun photo editing!