We've covered the free Aviary online image editor several times -- it is robust enough to work for most people who want to tweak their photos but don't require a full version of Photoshop or GIMP. They've just released a plugin for Android that brings the same functions to your phone or tablet so now you don't have to transfer the image to your computer before editing.
Keep in mind that Aviary is a plugin, not an app. This means that instead of launching from the home screen you will instead go into your photo gallery, pick a photo and select the 'Share or Edit' option -- Aviary will be one of the options.
All the usual tweaks are here: cropping (custom sizes and standard print sizes), adjustments of brightness/contrast/saturation/sharpness, flip or rotate the photo's orientation, red-eye removal, teeth whitening and blemish removal as well as more advanced effects like auto-enhance, night-enhance and the ability to draw or add text to the image. There are also a few effects packs available such as the Grunge effects and Nostalgia effects packs that add Instagram-like photo tweaks.
If you're an Android user and don't already have an image editor you should definitely consider giving this one a try, especially given Aviary's commitment to improving their online image editors.
Aviary Photo Editor (plugin) is a free download from the Android Market.