There are many more, perhaps niche or specialized collections out there- what are ones that open educators may not know about that they may find useful? Any reccommended site ought to clearly identify the licenses under which content is shared,
For example, look at the Open Peeps graphic library by Pablo Stanley
Use Open Peeps is a library of hand drawn vector parts that can be used to create character images for your projects, ones that focus on representation of diversity. Download existing “peeps” or download the full library to build your own by mixing and matching character elements that produce close to 600,000 possible combinations.
Everything is licensed Creative Commons CC0.
Try Open Peeps at https://www.openpeeps.com/
What special open content sources do you know of that are worth sharing?