Snapseed

Google acquired Nik Software in 2012 and turned their mobile app into the most powerful free photo editor on any platform. Snapseed's selective editing, healing brush, and perspective correction rival desktop software. The "Looks" system lets you stack and save complex multi-step edits. It processes RAW files. It has curves, HSL, and structure tools. And it costs exactly zero dollars. There is no catch. Google just gave away professional-grade editing to flex on Adobe.