Don't know about "cool" but I always end up going back to opengameart.org

While most of the stuff is one off assets that do not fit together there are also some nice sets by some creators such as Kenney or Emcee Flesher

Also the Liberated Pixel Cup (LPC) stuff is pretty nice.

Mind you I mostly just look for 2D assets.

https://opengameart.org/users/kenney

https://opengameart.org/users/emcee-flesher

https://opengameart.org/content/lpc-collection

For prototyping I usually use Kenney assets (from kenney.nl). Theres lots of assets for many types of games which lets you rapidly prototype and iterate without worrying about making assets first. For sounds you can use BFXR and just play around. Sometimes playing around with sounds can even lead to new feature ideas :)

I've used itch.io before, it's great! I even made a game using some free assets and AI: https://github.com/acatovic/gothicvania-codex-demo

itch.io is good for sprite assets and fonts I find.

Putting as much love into it as you can.