Open source texture art pieces hosted at external locations:

Animal texture by p0ss Animals image source
Bones texture by p0ss Bones image source
Brick texture by Tiziana Bricks image source
Building texture by Crack.com Buildings image source
Camouflage by Skorpio Camouflage image source
Concrete texture by Sindwiller Concrete image source
Crate by Luke.RUSTLTD Crate image source
Filth texture by TRaK Decals image source
Door by Saroman Doors image source
Fabric by Anna Paar Fabric image source
Fire by Lamoot Fire image source
Apple by qubodup Food image source
Grass by Lamoot Grass image source
Ground texture by qubodup Ground image source
Metal by TRaK Metal image source
Ornament by n3b Ornaments image source
Paper by Luke.RUSTLtD Paper image source
Road by yughues Roads image source
Rusted metal by p0ss Rust image source
Roof by Allan Zieser Roofing image source
Sign by Rafal Konieczny Signs image source
Sky by MiniBjorn Skies image source
Cactus by qubodup Vegetation image source
Car underside by qubodup Vehicles image source
Water by Rudy Phelippeau Water image source
Window by Jaycee Barratt Windows image source
Wood by Bart K. Wood image source
Hand X-Ray by Nevit Dilmen X-Ray Images image source

Signpost by Mumu About

This simple website serves as a waypost leading to freely licensed, legal textures for 3D modelling and graphical design needs.

More websites holding such art of various kinds can be found in a list on FreeGameDevWiki.

If you would like to get involved with the community or share your art, try OpenGameArt, their forum and IRC as a first stop.