Sweet spizz
I started with just a small tuft that came as a contaminant in a plant order. I wedged it into a crevice on a small piece of wood and 5 years later that one small tuft turned into this:
I've seen it used elsewhere to carpet wood, as groundcover and background as well. It works beautifully as groundcover as it looks like a clean miniature lawn. Starting out with a good amount you can easily cover a background or substrate and all you have to do is wait till it really takes hold, takes off and fills in the rest.
Take a look around the AGA contest pages. You'll find lots of inspiration there.
http://showcase.aquatic-gardeners.org/2007.cgi?&op=showcase&category=0&vol=0&id=101