Will add to the list as people submit content.

LemmaSoft Creative Commons Forum (lots of stuff here):
https://lemmasoft.renai.us/forums/viewforum.php?f=52&sid=70a6ce2c3f2a9e80d56292d...

Character Sprites:
https://itch.io/c/1115123/new-style-free-to-use-assets
https://itch.io/c/698965/yume-series-free-to-use-assets

Backgrounds:
https://tidalblossoms.itch.io/free-outdoor-visual-novel-backgrounds
https://noranekogames.itch.io/yumebackground