Here are three sites to use with your students if you are studying money.
#1 - The Lifespan of a Dollar Infographic
#2 - Festisite -Put your photo onto money. Look at money from around the world.
#3 - Presidential Portraits - Show what you know about the Presidents on money.
And a bonus:
#4 - A collection of links on Delicious by one of the people I follow, jmillot.