There are a tons of texture sites out there, but here is a handful of some of the best.
CG Textures http://www.cgtextures.com/
This is the first site I go to when looking for textures. Its well organized and easy to find exactly what you need.
Mayang Textures http://mayang.com
This site has tons and I mean tons of great textures. It doesn't look like much, but explore the site and you'll see what I mean.
Grunge Textures http://www.grungetextures.com
This site has a lot of really cool and fun textures. Not all are right for 3d work, but there are tons of cool shots.
Texture Warehouse http://www.texturewarehouse.com
This is a straight-forward site and with an esay to navigate texture search.
