Online APK Decompiler
http://www.decompileandroid.com/
http://www.javadecompilers.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
