Textures.com - Textures.com is a website that offers digital pictures of all sorts of materials.
Sketchucation - Sketchucation is a 3D SketchUp Community for students, professionals, designers, and hobbyists.
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.
CGTips - CGTips is a 3D Architectural Visualization learning platform for Architects or interior designers to build their skills in an interactive way.
Poliigon - A library of materials, and HDR's for artists including free textures that want photorealism.
3dxo.com - 3dxo.com is a great place for both professionals and hobbyists to find some really cool, free 3D models.