Snapseed: brilliant for quick on-phone edits but with the ability and range of tools to go much deeper.
Pixlr, a good alternative to Snapseed.
VSCO: one-click film looks
Afterfocus: select the focus area and defects the background after taking the shot.
Hyperfocal Pro: calculate the hyperfocal area for a given lens, aperture, and distance.
Photographer's Companion: a grab bag of tools including equivalent exposure, DOF, FOV, and exposure value calculators, ephemeral, light meter, and more.