Here's another that offers 100 high quality Twitter buttons:
http://www.hongkiat.com/blog/100-remarkably-beautiful-twitter-icons-and-buttons/
(page loads slow for me)

http://www.colourlovers.com/ - "a resource that monitors and influences color trends. COLOURlovers gives the people who use color - whether for ad campaigns, product design, or in architectural specification - a place to check out a world of color, compare color palettes, submit news and comments, and read color related articles and interviews."

http://www.tartanmaker.com/ - makes plaid designs

http://www.stripegenerator.com/ - a cool stripe generator

http://www.stripemania.com/ - another stripe generator

http://www.twitterpatterns.com/ - A variety of ready-made twitter backgrounds including Dots, Flowers, Odd Ball Patterns, Ornaments, Pink for October, Plaids, Solids, Stripes.

Make it at least 1280 by 1024 pixels and maybe even bigger, to around 1600 x 1200.

Here's some places to find great looking photos with "usage rights". I put MorgueFiles first because they have the most liberal usage rights with few restrictions:

http://www.morguefile.com/
http://www.sxc.hu/
http://www.realimagebank.com/
http://www.flickr.com/

Here's the Google search for "free twitter backgrounds":
http://www.google.com/search?hl=en&rlz=1T4GGIH_enUS274US274&num=100&q=free twitter backgrounds&btnG=Search

http://blog.webdistortion.com/2008/08/06/26-awesome-backgrounds-for-twitter-fanboys-and-girls/

http://tweetstyle.com/category/free-backgrounds/

Also, for cartoon-like backgrounds:
http://twittertreats.blogspot.com/

"Create free Twitter backgrounds

In less than 2 minutes... Brand yourself on Twitter by creating your very own Twitter background. Then, let us promote your Twitter Profile for you on the web.

Get Started Here or take a look below to see how it works."

http://www.twitbacks.com/