I may be technically off-beat here, but I don't use a wallpaper because of the memory it wastes.
Plain blue background for me...
I change colors often and on each machine I use, but it is almost always a plain solid color background for me, too. Only recently have I started using pictures of my daughter as a background, but even then I usually center it amidst the solid color.
I like the solid colors. Right now mine are 0x32325A, 0xFFFFC6, 0x800000, 0x9AD0D0 and 0x46007D.