I like having multiple currencies (copper, silver, gold, plat, etc) but mainly so it's easier to buy/sell. I'd rather type 300p than 300000000g.
Multi-level Resource Systems can be fun if it's used for a crafting system (as long as you don't go overboard like SWGalaxies did originally, i.e. RandomName1 Aluminum is different from RandomName2 Aluminum)
Crimson Moon has an interesting system in place - credits are the primary currency but gold, platinum, silver, and tokens are also collected and used for different things (gold is used for armor upgrades, silver for weapon upgrades, etc).