Freetweeterdesigner is the most complete Twitter background design tool i've seen. It lets you customize everything: add pictures and shapes, colors, text and different effects for the elements you added in your Twitter background. The resulting background look really professional and of course, Freetweeterdesigner is free (well, it's name already said it, but i had to mention it anyway) and you don't even need an account.
Try it out for yourself: Freetweeterdesigner
