"If the Internet teaches us anything, it is that great value comes from leaving core resources in a commons, where they're free for people to build upon as they see fit."
There are many free and open resources on the internet that can be very useful for designers. This non-exhaustive list provides links to some of the best and freest. How you can use each resource will depend on the license associated with it (e.g., some you need to cite, while others you don't; some can be used for commercial purposes, while others can't).
You should consult the license associated with each resource before using it so that you know what your legal and ethical responsibilities may be.
A List of Free Online Resources by Category
Explore the resources above and construct a short list of your top go-to resource for each category with a brief explanation.
Don't see a useful free resource here? Make a suggestion.