TwitBacks has a number of categories for backgrounds and lets you put in your contact details as well. They let you put links on your background to your social networks, and will help you promote your Twitter profile. They also offer a customized background for $99.

FreeTwitterDesigner also has free backgrounds. I really like their easy to use interface. These are straight graphics without any bells or whistles.

You can just use a graphic image to use as background as well.