Free asset sites that generally need no account to download. Licenses vary; most allow commercial use, some ask for attribution; so check each item's license before shipping. Tip: after importing an image, use the bg button to knock out its background.

Remember to credit your assets. Many of these ask for attribution. Keep a running list of what you use; asset, author, license, link; under Settings → Info & credits, and turn on the automatic credits screen so players see it. You can also add your own author name, website and collaborators there.

Images & photos

Background music

Sound effects

Game art, sprites & icons

Fonts