HDR mode was added to the iPhone camera back with iOS 4.1 but being less tech-savvy than I would like these days, I somehow managed to overlook it until quite recently. Last week, we took a holiday in Tasmania, providing some good opportunities to put it through its paces and I must say that I am impressed!
HDR stands for High Dynamic Range and takes three photos at different exposures before intelligently combining them. Sometimes it makes no difference. (Probably often but I only use it when I think it might help.) Sometimes, though the difference is quite dramatic as the following examples show. (Left, normal; Right, HDR.)
An already impressive camera, made even better.