How to decompile an APK or DEX file on Android platform?

Online APK Decompiler
http://www.decompileandroid.com/
https://www.decompiler.com/
APK Decompiler App for Windows
http://forum.xda-developers.com/showthread.php?t=2493107

Update 2015/12/04
ClassyShark you can open APK/Zip/Class/Jar files and analyze their contents. https://github.com/google/android-classyshark

Update 2021/1/28
https://ibotpeaches.github.io/Apktool/
https://github.com/skylot/jadx

I have created a tool that combines dex2jar, jd-core and apktool: https://github.com/dirkvranckaert/AndroidDecompiler Just checkout the project locally and run the script as documented and you'll get all the resources and sources decompiled.

You need Three Tools to decompile an APK file.

1. Dex2jar - Tools to work with android .dex and java .class files

2. ApkTool - A tool for reverse engineering Android apk files

3. JD-GUI - Java Decompiler is a tools to decompile and analyze Java 5 “byte code” and the later versions.

for more how-to-use-dextojar. Hope this will help You and all! :)