Adobe Photoshop Express
This app provides you with a free, fun and easy way to adjust the appearance of still images. The cropping tool makes cropping easy. You can adjust the exposure, saturation, tint and contrast, and turn your image into black and white or into sketch. You can sharpen or soften your image, change the border or choose a frame.
I like the ease with which changes can be made. Sliding your finger across the image while using a feature, such as contrast, allows you to control how much of that filter you want to apply. A number at the top of the screen changes so that you can quantify the amount of change you are making.
There's not much of a learning curve, but you do have to get to know each filter and how much change is produced by your finger movement. For a free app, PS Mobile does a great job at adjusting images.