As a frequent landscape photographer, I've enjoyed exploring HDR (High Dynamic Range) imaging for quite some time. With HDR, your camera takes multiple images, usually at different shutter speeds, to capture both bright and dark areas at their best. Then the images are combined with software. Although most of my work is with a Canon DSLR, I've been impressed with some of the iPhone HDR apps that can help you get some really nice pictures under difficult lighting conditions.
TrueHDR has just released a big update to its app, and now offers Geo-tagging support, better alignment algorithms and some improvements in the light metering software. I've tried the app, and I do like the quality of the images it produces. My only complaint is it takes a bit too long to merge and align the images. In my tests, True HDR produced an image in 24 seconds. Pro HDR processed the same scene in 12 seconds, and the Apple built-in HDR saved quite quickly ((2 seconds) but didn't look nearly as good as images from the other two apps.
TrueHDR (which requires iOS 4.0 or greater) is currently on sale for a limited time for US$0.99. If you're interested in HDR photography and want to go beyond the Apple built-in HDR, TrueHDR is worth a dollar.