Also ultramon lets you use one image as a wallpaper on both screens. Like I have mine set to 1600 x 1200 so I could use a 3200 x 1200 image across both screens. I know I can just cut the image in half and stick one half on each monitor but it would be nice not to have to do that.

I haven't personally looked for such software, but a cool multiple monitor / OS X trick is to cut several multiple monitor backgrounds in half, put them in two seperate folders in the same file name order (IE a1.jpg a2.jpg, b1 b2 etc..), and set the background to change on both monitors. Nice effect of both fading out the old image and fading in the new one.