Anything can be money if people put a value on it, and it works as a medium of exchange.

It is usual for money to change with the times. Typically, there is a dominant currency (such as Federal Reserve notes in the USA), then there can be all kinds of other types of money. In the USA, we tend to use only one type of money – the dollar, but there have been all kinds of money through history.

 Government
Alternatives

17th Century Gold, Silver, Copper Wampum, Beads
Colonial Paper Money Corn, wheat
Continental Dollars Tea, Sugar
18th Century Silver Dollars Hours of skilled labor
Gold Eagles Tobacco
Copper Pennies Iron, Steel
19th Century State Bank Notes Whiskey
US Notes (Greenbacks) Casino Chips
Gold Certificates Commercial Paper
Silver Certificates Company Scrip
20th Century Bus tokens
Federal Reserve Notes Store Coupons
Stamps
Military Scrip
Community Scrip
Baseball tickets
Airline miles/tickets
Phone minutes
Liberty Dollars
College credit hours
Disney Dollars
21st Century PayPal money
Facebook Credits
Virtual game money
Google credits
Net Metering (kwh)