Are "competing currencies" already allowed?

[I spent the winter writing songs about getting better]

I'm reminded of SPC and the rest of the braindead Libertarians proposing this, but actually it's already true. If I wanted to open a store that accepted only Monopoly money as payment, I could legally do so. It just wouldn't last long because the idea of "competing currencies" is stupid on its face.

And actually it has been tried recently. Remember this? http://en.wikipedia.org/wiki/Flooz.com