This is a specifically created Community Wiki which gathers resources for Blender and it has been approved by the Community.
Just write in the appropriate answer/section. If you have concerns, questions, post a meta question, so we don't clutter the comments, but you can link your meta question from the comments.
Questions regarding such resources are not allowed anymore, except for very specific and on topic requests (ask on Meta if you're unsure about your question). Follow the instructions made in the question about how to post, what can be posted, etc.
- Materials (also see: Any good free materials libraries online?)
- Textures and HDR environment maps
- Reference Images/Blueprints (also see: Where are some places to get reference images for modeling?)
Also see the beginner's reference.