Android developers - Everything you need to get started with Android development.
Stack Overflow - If the Android Developers page gets tricky to understand, this will clean up any thing you need help with
Android Studio - The only IDE we use for developing our Android apps and courses
Material Icon's - An amazing site with user submitted icons for use in an Android app.
Blender - A powerful free and open source 3D modeling program. Steep learning curve but well worth it if you put in the time to learn.
Blender Artists - Forums for Blender. Everything from inspiration to help with learning.
Blender Guru - Hands down THE best Blender tutorials on the web. And 100% free.
GIMP - For editing and adjusting icons/images/ads etc.