I like two apps, one a camera app, the other an editing app:

1. My favorite full featured editing tool is Snapseed. This allows you to take images from your Camera roll and edit with impressive control. It features selective adjust, allowing you to touch where you generally want edits, and offers edits like brightneess, contrast, whitebalance, and saturation. It also includes sharpening control, and of course a bunch of filter effects. My favorite is a finely controlled tilt-shift effect that is fun.

2. Camera+: not an editing app per se, but a fantastic camera app that has some editing function. It has many features not found on the native app, including a pseudo-stabilizer function that works well. Camera+ includes many fun filters and effects. However, photo 'editing' is really limited to crop, as it resorts to 'scenes' rather than white balance, brightness, etc. Probably the best feature is that it has its own photo storage, independent of the Camera Roll, so that you can choose which images to save to the camera, and therefore the new iCloud PhotoStream.

3. Adobe Photoshop Express: this is a fairly limited app, but with some nice 'before and after' features, and before Snapseed was my goto app for rotating images. Its free, so you get what you pay for. It will charge you for more features (as will the others) and also encourages you to get an Adobe account for photosharing. (no thanks)