You may take a perfect picture every time. I sure don’t. I’m happy to have found SnapSeed, a dead simple photo editor. You can do all the things you expect: crop, adjust colors and brightness, sharpen, straighten–and do that all selectively. You can add a frame and various filters (even a vintage filter for you Instagram fans). Cartoon-like overlays show you what to do in each case.
You can 
then compare your current view with the previous view before you decide to apply your change.
Once done you can send your image to Google+, Email, Twitter, Facebook, or Open it in any app you have that can use images (including DropBox), or you can save it.
Best of all? The app used to cost $4-5 but is now free!