iOS: You don't need to touch up your selfies in Photoshop before posting them to Facebook. Facetune is a powerful portrait editor that can tackle just about any issues with a portrait right on your phone.
The idea is similar previously-mentioned Beauty Box, but Facetune is more powerful and intuitive. The app includes built-in brushes to whiten teeth, smooth wrinkles and enhance details. You can even reshape parts of the photo to improve a smile, or un-furrow an eyebrow. At any time as you work, you can tap and hold a history button to see the original photo overlaid over your current view, giving you a great idea of how far you've come.
My favourite aspect of Facetune is its approachable implementation of the clone stamp tool (called "Patch" here) to cover up blemishes and zits. You simply tap the point you want to fix, then tap a source point nearby. Then you can pinch to change the size of the effect, and rotate with two fingers to turn the stamp on its axis to make it look more natural.
Once you've fixed the photo itself, you can apply a number of flattering filters, and share it to various social services, or even other iOS photo editors. As far as photo apps go, Facetune is a little niche, but it's incredibly well-suited to its purpose.