LAST EDITED ON Apr-05-09 AT 05:37 PM (PST)
 
Here's a page that lists "181 Free Twitter Buttons, Badges, Widget and Counters to Help You Find Followers".
http://www.twitip.com/181-twitter-buttons-badges-widget-and-counters-to-help-you-find-followers/

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)

Here's some resources to help create different "tiled" backgrounds:

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.

LAST EDITED ON Apr-05-09 AT 11:23 AM (PST)
 
You can also use a larger pic/graphic as a Twitter background.

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/

LAST EDITED ON Apr-05-09 AT 11:29 AM (PST)
 
My favorite Twitter background generator:
http://www.mytweetspace.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

Here's a cool collection of 26 twitter backgrounds...He even gives you the color codes to use to make sure your twitter page is "color coordinated".

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

Some more FREE twitter backgrounds:
http://twitterbacks.com/

LAST EDITED ON Apr-05-09 AT 05:34 PM (PST)
 
Here's a few free backgrounds...Not a big selection, but high quality.

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

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

Here's another very good background generator:

"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/