Snapseed is a complete and professional photo editor, developed by Google.
Once installed, you can pick up any photo from your Gallery and work on it in a professional manner. You get 29 Tools and Filters, including: Healing, Brush, Structure, HDR, Perspective (see list below). You can save your personal looks and apply them to new photos later. All styles can be tweaked with fine, precise control.
Cropping and rotation is a breeze. You can use the Brush to selectively retouch exposure, saturation, brightness or warmth. Once you are done, you can add both stylized or plain text onto your photos. And, of course, you can use Double Exposure to blend two photos, choosing from blend modes that are inspired by shooting on film and by digital image processing
All in all a professional photo editor at your fingertips, for free!