Twitter Background Designer: Echos of Photoshop
Easily the best Twitter background designer I've found (not that I've ever spent much time looking). There aren't many themes to get you started, but if you already have a design you want to convert to a Twitter background, there are quite a few options to help you get a unique look. You can upload images, choose from a variety of fonts and colors and even choose from a shapes menu.
The main design interface.
The Text Tool.
