APK Editor is a powerful tool that can edit/hack apk files to do lots of things for fun.
It can help us to do things like string localization, background image replacement, layout re-architecting, and even ad eliminating, permission removing, etc. What it can do depends on how you use it. However, to use it well, we need a little bit professional skills. Don't be afraid, some examples are given in the help page.
(Note: Manifest editing is not supported in free version)

From version APK Editor 1.9.0:
(Android 3.1+)
Latest: 1.8.15 on September 7, 2017
(Android 3.2+)
Latest: 1.9.0 on September 30, 2018
All versions
September 30, 2018
November 8, 2017
October 3, 2017
September 7, 2017
August 7, 2017
June 14, 2017
April 4, 2017
February 19, 2017
September 30, 2016