Free Assets

2D sprites:

2D Mega Pack

Unity’s Asset Store, filtered by free and 2D

Itch.io: top free game assets tagged 2D

CraftPix, filtered by free

Kenney, filtered by 2D and UI/Fonts

Open Game Art

3D models:

Unity’s Asset Store, filtered by free and 3D

Itch.io: top free game assets tagged 3D

Sketchfab: downloadable models, sorted by most liked

Brackeys’ Dev Assets

Kenney, filtered by 3D

Blend Swap

Animations:

Mixamo

Music & sound effects:

Sample Focus

Freesound

Kenney, filtered by audio

SoundCloud (use an online converter if necessary)

YouTube has a lot of royalty-free stuff too.