Android/iOS: Aviary already offers a great image editor for your web browser, but it now has an app for your mobile phone too. It provides an extensive toolset for improving your photos, plus some fun add-ons that you can purchase for a few bucks.
Aviary can take your photos and whiten teeth, remove blemishes, sharpen dull areas, improve contrast and saturation, brighten or darken the image, correct backlighting, fix poorly lit night shots, balance colour and tone, and comes with a general and automatic ENHANCE feature. You can also draw on your images, add text or apply various stickers. All of this stuff is free, but Aviary offers image effects and additional sticker packs at a small cost. If you're doing any actual image-editing on your phone, the free tools should do the trick. If you want some of the fun stuff, however, you'll have to make a small contribution via an in-app purchase.
Overall, the app is a very comprehensive photo editor. It's definitely worth a look, and it costs you nothing to download.