ModDroid is the best way to modify Android apps without jailbreaking. We've written a Mac app that interfaces with Android at the USB level to download an app, decompile it, apply a patch that extends the app's functionality , recompile it, and reupload it back to the device! Our proof of concept is the creation of a special Valentine's Day patch which makes the Facebook Messenger app themed around the upcoming holiday.
This new approach allows us to extend the functionality of apps like Facebook Messenger and Yik Yak without having to compromise the phone with a jailbreak. It also is simple enough to use that those who haven't used a command line before can jump right into the world of personalizing their mobile experience with new skins and features.
With more work, we'd like to see ModDroid become advanced enough that anyone can build their own modifications (mods) simply by picking and choosing colors and images with easy controls. We think that ModDroid could catalyze a new kind of app store where enthusiasts share their creations with others for money. We'd like to think that ModDroid can do for mobile phones what mods did for early PC-based computer games.