There's no shortage of photo editing software programs or cool iPhone camera apps, but Photoshop is arguably one of the best around. The free (yes, free!) app lets you perform basic photo editing tasks: crop and rotate, adjust colors, add tint or switch to black and white, and add filters or effects like vignette blur and warm vintage. Creating an account on photoshop.com and store up to 2G of photos that you can access anytime straight from your phone.
You can download the Photoshop app from the App Store.