Cows might have served well as currency for primitives like the Etruscans, but can you imagine using them today? I took a bus this morning, the fair was 1.10 and I only had paper money so they ripped me off 90 cents. But if I was an Etruscan, they would've taken my whole cow!
More likely the "fare" was 1.10, but whatever :-) There's a nice changemaking application for the Palm Pilot that lets you input the amount of money you want to pay somebody and the size of payment you can make, e.g. $1.10 and $5. It splits a circle into appropriately sized pie slices, you spin the dial, and either pay or don't pay. Of course, getting a bus company to trust your Palm Pilot not to be running a rigged version may be tougher than getting your mathematically inclined friends to accept it :-)