Free to use Resources
Images
Unsplash (No permission needed, free to use for commercial and non-commercial purposes)
Public Domain Pictures (Public Domain, No permission needed, free to use for commercial and non-commercial purposes)
Pixabay (No permission needed, free to use for commercial and non-commercial purposes)
The Gender Spectrum Collection (Creative Common)
OpenMoji (Creative Common)
Clipart
Openclipart (Public Domain, No permission needed, free to use for commercial and non-commercial purposes)
Publicdomainvectors (Public Domain, No permission needed, free to use for commercial and non-commercial purposes)
Noun Project (Icons, and also photos) (Attribution Required - Non-Commercial only)
Storyset (Clipart, modifiable directly online) (Attribution Required - good resource when you need to tell a quick story)
FlatIcon (Icons) (Attribution Required)
Audio/music
Audionautix - Mostly music (CC 4.0 BY, No permission needed, free to use for commercial and non-commercial purposes as long as you credit the author)
Freesound - Sounds (Various licenses, check each sound)