Anatomy / Fundamentals

• simple forms 3d models to copy, useful for rotation exercises
• planes of the head reference 3d models, masculine / feminine versions
• inner body 3d atlases for human muscles and skeleton
• https://x6ud.github.io: animal skulls 3d models

Poses

• posemaniacs: 3d viewable poses
• adorkastock: free pose references
• true ref : free/paid pose library (free linked)
• cubebrush: you can buy references for $$$, has some freebies

Animation

• bodies in motion include motions and heads shot from different angles, and also all the animations from the Muybridge's books (only few items are free)
• sakuga booru
• you can also use animations on mixamo as reference

Topics

• inaturalist : a community curated archive of animal and plants pictures
• Andrew Naylor's travel and transport photos : trains and planes
• Steam Trains Galore
• for landscape from above, go on youtube and take some snapshots of drone footages

Royalty Free Photos

• pixabay
• unsplash

Archives

• artvee : curated archive of public domain classical and modern art