Textures.com – Originally CGTextures.com (back in 2005), offering 15 free images per day out of a library of more than 125,000.
Pexels.com – Offers free stock photos and textures out of a library of several 100,000 photos.
SolarSystemScope.com – a solar-system-related texture website by space enthusiasts based on data from NASA.
TextureKing – a collection of a few hundred textures.
SweetHome3D – sweethome3d’s list of websites for textures.
Sirious sds’s texture packs – contains several texture packs, some of which are free.
Want your textures added to this list for free? Contact me.