Snapseed is one of the best photo editors available for Android, especially considering it's completely free. A year and a half after the app surpassed 50 million downloads on the Play Store, the app has now achieved over 100 million installations, according to its Play Store listing.

I'm not aware of any phones with Snapseed pre-installed, so this is definitely a more impressive benchmark than apps like YouTube or Facebook achieving large numbers. If you haven't already tried it out, you can download Snapseed from the link below.