HDR, or high dynamic range, can add clarity and depth to your iPhone photos. The iPhone's native camera app has an HDR option (see Sharon's post about it and HDR photography), and it does an admirable job of filling in the highs and lows of a photo. I found that I was able to achieve even better results with TrueHDR, a $1.99 app.
When you launch TrueHDR, you're given four options for creating an HDR image: Auto Capture, Semi-Auto Capture, Manual Capture, and Choose Pictures.
With Auto Capture, you simply tap the camera button and hold your iPhone steady as TrueHDR takes three photos. After a few seconds, tap the Merge button and TrueHDR will create one image. SemiAuto Capture lets you select two exposure points (one in a dark area and the other in a bright area) before TrueHDR takes two photos, which you can then merge together. Manual Capture is similar to SemiAuto, but it snaps a picture after you select a dark exposure point and another after you select a bright exposure point instead of snapping two shots in succession after you choose both points. The fourth option lets you select two photos (of the same scene, each shot with a different exposure) from your Camera Roll and have TrueHDR merge them together.
After you merge two photos together, you can save it to your Camera Roll, trash it (by hitting the Home button), or share it via e-mail, Facebook, or Twitter. There are also two editing options. The first gives you sliders to adjust the warmth, contrast, saturation, and brightness of a shot. And by tapping the Fx button, you can apply one of seven filters. There isn't an original option among the filters, but to opt out after browsing the filters, just tap the selected filter to unselect it and return to your original photos. At any rate, the photo filters feel like a departure from the HDR mission of the app, almost as if the developer saw how much Facebook paid for Instragram and decided to toss in handful just in case Zuckerberg was still on the prowl for another billion-dollar photo app.
In my experience, I found that TrueHDR did a slightly better job with HDR photos than the HDR option on the native iPhone camera app. If the iPhone's default setting is good and its HDR setting is better, then TrueHDR is best. For most users, however, the iPhone's HDR option will suffice. Plunk down your $2 only if you are an enthusiast.