DeviantArt: This is usually a good first stop when looking for some texture inspiration. It's nice because a lot of the textures on the site can't be found anywhere else.
Flickr: There are so many different textures on here it can be overwhelming at times, but a very good source of ideas and inspiration.
CGTextures: This site hosts all kinds of textures in an easy to browse format.
Zen Textures: A large assortment of high resolution textures.
QTextures: An ever growing stock pile of amazing textures.
Lost & Taken: A nice variety of detailed and colorful surface textures.