One guy named Bernard von NotHaus created the "Liberty Dollar" and is now facing a 45 year prison sentence. After a little googling, it's unclear why exactly he is being punished, but this how one article summed it up:
"Congress has exclusive power to coin money in the U.S. and to regulate its value, according to the Treasury Department."
This makes me wonder: is it illegal to use non-US coins for transactions in the US? Like if I use Euro-coins? (I say coins because there seems to be a specific prohibition on issuing "coin" and not other forms of currency unless they resemble US dollar bills.)
Counterfeiting, mostly. The coins he made looked like normal US coins, except that the presidents were swapped out for Ron Paul or other anti-establishment figures.
"Congress has exclusive power to coin money in the U.S. and to regulate its value, according to the Treasury Department."