Applications using the Index Card metaphor are available for the Android platform as well. There are Cardboard Index Cards and CardScape, for instance. They seem to be doing essentially the same thing as the iPad applications.
Supernotecard has for a long time done the same for the desktop.
As I said before, I am not sure whether this skeuomorphic approach is always the best approach. Do we really have to move around objects that look like index cards in order to outline material. Clearly not! But in this case, I am willing to admit that it might be helpful. Though the sticky notes analogy might do just as well. See Stickysorter for Windows, for instance.