There are a variety of different backgrounds that can be used for the web. Backgrounds that are way too busy are not appropriate as the text does not show through. In addition, if you want any of your pages to be printed, remember that a colourful background will interfere with your text.
To give you some ideas, I have prepared a small Flash side show to demonstrate some of the different backgrounds you could use. Please also bear in mind that copyrights apply to many of the so called "free backgrounds" on the web.
Another site where you can see some more backgrounds is http://www.backgroundcity.com/
Note this quote from the site "Welcome to BackgroundCity.Com...
your best source for backgrounds, all original and absolutely free!"
Bottom line, read the fine print before you decide to use images from the web, in many cases, they are free to use as long as you add a banner to your site, and as long as you have a personal site. The banner would be something like the one below: