unDraw:

You can use the illustrations published on unDraw for personal and commercial projects, without the need to include attribution.

Unsplash:

All photos published on Unsplash can be used for free. You can use them for commercial and noncommercial purposes. You do not need to ask permission from or provide credit to the photographer or Unsplash, although it is appreciated when possible.

Humaaans:
Free for commercial or personal use as granted by Pablo Stanley.

UnDeck:
Free for commercial or personal use as granted by KP