Snapseed is a great all-around photo editor. A long time staple for Apple devices, Google purchased Nik Software and released the same feature-packed image editor that users know and love for Android. It's easy to use, and it makes taking and sharing pictures with your Android even more fun.
After you've taken a picture and shared it with Snapseed, you can use simple automatic tools that will make your picture better, or dig deep into all the options and make adjustments by hand. Just about anything is possible, from simple adjustment like cropping and zooming, to advanced features like precision selective adjustment of contrast or color balance. Fun tools like frames and filters — you have to give the Retrolux filter a try — round things out. No matter what you're looking to do with your pictures, chances are you can do it in Snapseed, and once you're finished sharing it on Google+ is just a button away.
Snapseed is completely free, and one of the apps all the editors here at AC use.