http://www.free-backgrounds.com

   The sites main theme is backgrounds. They have tons of professionally made backgrounds which will work well with just about any website. We've heard a lot about this website, and would strongly suggest you take a look.

http://www.andyart.com

  This site has all types of free graphics including a very large free background section. Definately worth taking a look if you haven't already run into this site.

http://www.topazdesigns.com/~klm

  This site has a large collection of backgrounds, which cannot be found anywhere else on the web. The backgrounds are sorted out by theme. Topaz has nature backgrounds, cube backgrounds, large backgrounds, and a miscellaneous catagory. Some backgrounds may not work to well with internet sites, but there are some which'll work well.