Textures is always useful for designer 
, now you can bookmark this one too – Texturise.Com . They have nice collections of textures and categorized it by Clouds Texture, Concrete Texture, Food Texture, Grunge Texture, Paint Texture, Paper Texture, Plants Texture, Plastic Texture, Rocks Texture, Rust Texture, Water Texture and Wood Texture. All of textures are free for personal and commercial use. So it is safe to use it for your commercial projects.
It seems the site is still grow up, and I hope the collections will be grow too. The one of my favourite is their collections on concrete textures, they have one concrete texture that really I like and match with what I need for grunge style of design.
These is some of texture samples from Texturise.com :
Cloud Textures
Texturise Concrete Texture
Texturise Cardboard Textures
http://www.assistcg.com/ has also great cloud textures – panoramas
http://www.LoveTextures.com Great site with over 200 Hi resolution textures and backgrounds for print or on screen, Check it out