Reference imagery on a variety of topics.
Game Art Reference
Game Art Examples List: game art dumps on the Polycount forum.
Level Design Reference Database: a searchable collection of game screenshots with assigned tags that describe the images’ content.
Forum Reference Threads
CategoryReferenceAnimation for motion reference links
Reverse Image Search
Search for images similar to a source image.
RevIMG - search within specific subjects: flags, famous paintings, symbols, monuments, etc.
TinEye - reverse image search
WhatTheFont - reverse image search for fonts
cgtextures.com Free up to a daily limit, subscription fee thereafter.
mayang.com Free up to a daily limit.
lemog.fr Free. Most are tiled.
free-textures.got3d.com Free for a subset of the whole library, the rest available for a fee.
newtek.com/freestuff Free but rights-limited. Requires registration.
texturewarehouse.com Free but rights-limited.
environment-textures.com Subscription fee required. Part of the 3d.sk family.