https://code.google.com/p/android-apktool/

It is a tool for reverse engineering 3rd party, closed, binary Android apps. It can decode resources to nearly original form and rebuild them after making some modifications; it makes possible to debug smali code step by step. Also it makes working with app easier because of project-like files structure and automation of some repetitive tasks like building apk, etc.

It is NOT intended for piracy and other non-legal uses. It could be used for localizing, adding some features or support for custom platforms and other GOOD purposes. Just try to be fair with authors of an app, that you use and probably like.

Features

• decoding resources to nearly original form (including resources.arsc, XMLs and 9.png files) and rebuilding them
• smali debugging: SmaliDebugging
• helping with some repetitive tasks

Requirements

• JRE 1.6 (Java Runtime Environment)
• aapt command in a PATH
• basic knowledge of what is SDK, aapt, PATH, smali and Google search engine may be useful

Installation for noobs

• Windows:
  1. Download apktool-install-windows-* file
  2. Download apktool-* file
  3. Unpack both to your Windows directory
• Linux:
  1. Download apktool-install-linux-* file
  2. Download apktool-* file
  3. Unpack both to /usr/local/bin directory (you must have root permissions)
• Mac OS X:
  1. Download apktool-install-macos-* file
  2. Download apktool-* file
  3. Unpack both to /usr/local/bin directory (you must have root permissions)

Installation of framework files

• See FrameworkFiles

Usage

• Open terminal/command line and type "apktool <enter>". Then you should see usage help.