Even though I do a lot of high end photo work, I still enjoy using the iPhone's camera, and i was especially happy to see Apple add HDR  capabilities to the iPhone 4  . HDR lets you get good pictures under challenging conditions when there is a mixture of very bright light, like the sky, and objects in shadow.
I've also checked a lot of the third-party HDR apps, and most exceed the quality of the built in HDR feature. Today only, HDR Fusion  is free, so it's a nice Friday present for those thinking about going beyond the Apple HDR and trying something a bit better.The app is normally US$1.99.
HDR Fusion grabs two images, one underexposed and one overexposed, then tone maps them to a combined exposure that doesn't blow out the bright parts or lose the shadows. In my quick tests, the app worked well in both the auto mode and manual mode. If you are manually shooting, you drag two markers around the field of view, one to the darkest shadowed area, the other to the bright highlights.
Processing is faster than most of the other third party apps I've tried, but not as fast as Apple's almost instant implementation. Picture quality is pretty good -- exceeding the Apple feature in some of my tests, in others about on par with it. In general, I still prefer Pro HDR  on the iPhone, but at least today there is a free alternative.
HDR Fusion also includes a self-timer, which is a handy feature. The app is a pretty small download, just over half a megabyte. Free is good, so give HDR Fusion a try and save yourself two bucks. The free offer will expire at different times in different time zones, so be aware that the offer will likely end at midnight EDT in the U.S.
In the gallery, I've compared the app to the built-in Apple HDR and Pro HDR, which is still my go-to app for HDR on the iPhone.