What it does: It's a simple calculator that helps you determine specific depth of field measurements as well as other tricky items like exposure reciprocation and manual flash exposures.
Why it's useful: If you have been a good little photography nut, you have a lot of the formulas committed to memory, but it's a lot faster this way. Plus, it comes with some handy reference docs like a glossary and a zone system reference. Less time doing math and more time triggering your shutter is always a good thing.
What it does: A handy interactive graphic displays vital information about the sun and the moon. Keep track of their position in the sky, intensity and the exact time at which they will rise and set.
Why it's useful: Catching magic hour--the time in the day where the light produces the best images--can be tricky if you're not exactly sure when it is. Having the right light can even make iPhone photos look good, but this app is handy for photographers who are travelling on assignment, too. There's nothing worse than waking up to shoot and realizing that the sun is already glaring straight down on the top of your subjects heads.
Nikon My Picturetown Free
What it does: Get pretty much full access to Nikon's photo storage and sharing community.
Why it's useful: You don't have to be a Nikon user to take advantage of their surprisingly robust community. You can upload your iPhone photos or grab your already uploaded photos for viewing on your handset. Since it was built by Nikon, there's no real worry about compatibility issues or glitches. It's also compatible with the iPod touch even though it's tragically camera-less.
What it does: Stitch together your iPhone photos without uploading them to your PC.
Why it's useful: Weave together a couple of 1,600 x 1,200 pictures and you end up with a pretty interesting result. You can do up to 30 images, but when you get that high, it gets pretty difficult to manage. Anything less than 8 and you should be fine.
PanoLab in the App Store
What it does: One of the most full-featured image editors available for the iPhone lets you crop, sharpen, rotate, straighten and adjust crucial values like gamma, saturation and color temperature.
Why it's useful: It's one of the top-selling photo apps in the iTunes store, probably because of all the options it gives you. There's even a levels adjustment tool with a histogram. Of course, seeing the histogram for an image taken with the iPhone might be a little scary. Don't worry, you can fix it.
Get it in the App Store