Free / no Copyright / CC-BY
Here are some links to archives which offer textures "without any copyright", compatible CC licenses or public domain. Attention: If you want to extend this list, note that "free" or the lack of any copyright message doesn't mean anything. To be public domain or "without any copyright", it must be explicitly noted.
- Flickr - mixed CC licenses
- OpenGameArt (3400+) - various open source licenses
- LuGher Texture (3400+) - free to use in personal and commercial projects
- Accustudio (~1000) - free of any copyrights
- The Texture Barn (~500) - "Royalty-free textures for computer graphics, illustration, game design and everything else"
- Ben Cloward (~300) - free but author would like to be notified where it is used
- Pixar - CC-BY 4.0
Mostly free with own license
However, if you find really good textures there, it doesn't hurt to ask the original authors whether it is possible to grant permission to use a certain texture under a CC license that is compatible with open source. Same holds for images collected for example from flickr that are not licensed under a CC license.
- CG Textures (30000+)
- Image After (~10000)
- Mayang's Free Texture Library (3800+) - max. 30 textures may be used
- texturemate (4400+)
- 2Textured (2700+) - "The textures may be used freely in commercial or personal works but reselling or redistributing of the textures on other texture sites is forbidden"
- Freetextures.org (~1000)
- Texturez (800+)
- Free Stock Textures (800+)
- Free Texture Site (600+)
- Zen Textures (450+)
- Noctua Graphics (400+)
- Texture King (300+)