There's the Triganic Pu...

The Triganic Pu has its own very special problems. Its exchange rate of eight Ningis to one Pu is simple enough, but since a Ningi is a triangular rubber coin six thousand eight hundred miles along each side, no one has ever collected enough to own one Pu. Ningis are not negotiable currency, because the Galactibanks refuse to deal in fiddling small change.

-- Douglas Adams