This one really is different.

If you're creating an app, you are in constant need for a lot of elements, tips and sources. Here's a list of great sources, most of them FREE, to use when creating an app:

Design & UI guidelines and tools:
Adobe Color Wheel
0 to 255
ui8
Flat UI Design

Free icons:
Flat Icons
Icon8
Icon Finder

Background/images/photos:
PixBay
DreamsTime
Free Digital Photos
Free Images
Stock Vault
500px
Unsplash
Pic Jumbo
Canva
Creative Commons

Photo Editing:
Pic Monkey
Pixlr
Fotor

Icon and screenshots re-size & design tools:
Angry Marmot
App Icon Template
PlaceIt
Mock U Phone
AppsBuilder’s App Screenshot Templates

App promotion video:
PowToon
Make Web Video
FlixPress

ASO:
AppTweak
Sensor Tower
AppBot
SearchMan
AppBrain
XYO
App Annie

A great list, with incredible tools, many of them at zero cost. Check them out.

Full article can be found here:
http://blog.apps-builder.com/app-marketing-hacks/