Update: While the developer of HDR told us the app would be free for some time, unfortunately our post was published the same day the app returned to its $1.99 price. We apologize for any confusion.
HDR is free for a limited time. The app takes two pictures, one light and one dark. It then aligns and blends them into one image with wider dynamic range. Unique to this app is a feature that shows you 4 options for the final image, labelled Auto, Optimized, Vivid, and Contrast. The app also allows you to bring in two images for processing that are already on your camera roll. Native resolution of your photos is maintained, which is a positive feature.
In my tests I found the app to be very fast at rendering the new image. The blending options make it more flexible than the Apple provided HDR feature. What the app lacks is the extended dynamic range of some of its competitors. That's easily tested by shooting from an interior room toward an open window. The HDR app loses a lot of detail in the shadows, while some of the other HDR apps don't. You can see some examples of this in the gallery attached to this article. I compared HDR to the Apple HDR feature, Pro HDR, which sells for US$1.99, and the $1.99 HDR3.
All the apps had strengths and weaknesses. ProHDR and HDR3 could see more detail in the shadows. ProHDR also did better on keeping hot spots from appearing in the sky. The Apple HDR feature also rendered the skies more smoothly. Apple does better at this because it doesn't push the HDR rendering as far as some of the other apps. Of course that can also be a negative. I didn't change the saturation on any of the images, and used the auto mode on the HDR app.