They're not Federation credits or gold-pressed latinum, but if you're in the Pacific island nation of Tuvalu, you can use these coins as legal tender. On one side of each dollar coin is the face of Elizabeth II, the Queen of Tuvalu. On the other is either Captain Kirk or the USS Enterprise. The Perth Mint of Australia offers these collectible coins made of almost pure silver.
P.S. If you know of a bank that will exchange quatloos for US Dollars, please let me know promptly. I need a good rate or I'm going to take a big hit unloading these thralls.