Textures.com - Textures.com is a website that offers digital pictures of all sorts of materials.
CGTips - CGTips is a 3D Architectural Visualization learning platform for Architects or interior designers to build their skills in an interactive way.
Poly Haven - Poly Haven (formerly known as Texture Haven) is an online website where you will find scanned textures without any cost and that are of high quality.
Sketchucation - Sketchucation is a 3D SketchUp Community for students, professionals, designers, and hobbyists.
Poliigon - A library of materials, and HDR's for artists including free textures that want photorealism.
Quixel Megascans - Megascans is the world's largest 3D asset scans library full of high-resolution, PBR-calibrated surfaces, vegetation and modular scanned assets.