Photoshop Express

• Developed by Adobe, Photoshop Express brings a host of image manipulation features seen on the popular desktop software to the iPhone. Photoshop Express offers a basic, yet powerful editing package with the primary purpose of improving the quality of photos captured by the iPhone and pictures saved from emails or the Internet. Light levels, color tables, contrast, shadows and other essential adjustment options are available, and you can crop, rotate, resize and physically manipulate your pictures quickly and efficiently. Photoshop Express is a free application, without any operational restrictions.
  
  

Filterstorm

• Although Filterstorm offers photo improvement functions comparable to Photoshop Express, it also includes a host of more artistic functions that change and manipulate your original iPhone pictures. Filters are applied in a controlled manner, with no permanent changes taking place until the "Apply" button is pressed. Filterstorm records all your actions as you perform them, allowing you to undo steps taken during editing or to apply the same steps to more pictures, recreating the same effects. Altered images can be saved to cloud storage services, or as a new picture directly on the iPhone.
  
  

PhotoPal

• The PhotoPal application offers a relatively basic range of image manipulation tools, with a particular emphasis on cleaning and sharpening your iPhone's pictures. These tools are particularly powerful, allowing you to significantly reduce noise on a grainy picture without losing quality or detail. Its zoom and sharpen functions make it easy to crop and resize certain areas of a photo, and to enlarge them while maintaining the detail and quality. Altered pictures are saved to the iPhone or uploaded directly to online image storage services and social networks.
  
  

SketchBook Mobile

• Although SketchBook Mobile can import your iPhone's photos, its primary functions are more artistic than technical. SketchBook Pro is a drawing package that allows you to paint, airbrush and draw your own digital art, or load an existing image -- such as a photo -- to use as a background canvas that you can paint over. The app uses layers, which are transparent images piled on top of each other, allowing you to work on an upper layer without making permanent changes to a lower one.