[The Creatives Club] Week 1 - Post your links, resources, tips, tricks and tutorials in here! Check the Guidelines & FAQ before you post! Guidelines and FAQ - justpaste.it
Textures - Free Textures - 3D Textures, Photoshop Textures is brilliant for realistic textures, seamless and non seamless, some comes with alpha textures too for plants 
Cyclic Gearz - This channel set has as many channels as I could find that do game related or creative related tutorials
Home - Death to the Stock Photo Free commercial use stock photos via email every month.
Home Free software to pose a model for the purposes of figure drawing.
Texturelib - free texture library - commercial use and free - you have 10 downloads a day. Realistic Textures
Free Textures from TextureKing free stock photos and textures, commercial use - mostly non seamless.
Retro Vectors - Brilliant site specialising in vintage and retro vectors - free for commercial use. Also has some brill tutorials too!
The Top Ten Tips of Texturing | CGSociety Article talks about technical aspects of texturing related to PS, but I am sure the same things could be translated to GIMP or something!
600 High Resolution Textures More textures! You can download individually for free, or support the creator by downloading all at once for $25 (Link sent to me by Kei)
texturer.com/ More textures, free, commercial use!