Freebbble!

Available for iPad and iPhones. Direct download links to iPad version, go to the Dribbble post for more alternatives.

Quattro Hot and Quattro Electric (Mobile) is a free Wallpaper design resource made by Dribbble user Marc Edwards Bjango. As far as I can tell, its license is Unknown.

Helpful hint: Dribbble users are nice. If the license is unknown, it is usually because they forgot to mention it explicitly; not because they were being stingy. Ask nicely and they will probably allow you to use it.