Mayang’s Free Texture Library has over 3400 free to download, free to use, high-resolution textures.
[Edited Feb 20th 2008] The website seems to be offline. That’s too bad!
[Edited Feb 29th 2008] Mayang is still offline. Here is at least a bit of a replacement: http://cgtextures.com/
[Edited Mar 4th 2008] This blog entry at Photoshopblog.net lists a number of good and free texture websites.