Androguard is mainly a tool written in python to play with:
Androguard is available for Linux/OSX/Windows.
The Android Decompile is a script that combines different tools for successfully decompiling any Android package (APK) to it's Java source-code and resources (including the AndroidManifest.xml, 9-patches, layout files, etc.).
This project makes it easy to convert an installed Android app that was built with the Android SDK into readable source files.
ClassyShark is a handy browser for Android executables. It has clients for both Android (apk) and Desktop (jar). With ClassyShark you can open APK/Zip/Class/Jar files and analyze their contents.
Enjarify is a tool for translating Dalvik bytecode to equivalent Java bytecode. This allows Java analysis tools to analyze Android applications.
JAva Decompiler is not maintained decompiler for the Java programming language, but it is still a powerful tool.
This is a Decompiler that extracts the source code of an Android application (including XML files and image assets). Works directly from your android device.
smali is an assembler/disassembler for the dex format used by dalvik, Android's Java VM implementation. The syntax is loosely based on Jasmin's/dedexer's syntax, and supports the full functionality of the dex format (annotations, debug info, line info, etc.)