Today, the only notes issued are Federal Reserve Notes in denominations up to $100. However, U.S. Government currency was not so simple in the past. Before the late twentieth century, a person's wallet might hold five or more types of currency in denominations from three cents up to $10,000. One could even come across some notes not issued by the U.S. Government.
The links on this page take you to descriptions of the more unique currency found today and yesterday: seldom seen obsolete forms of U.S. Government currency, rarer types of current Federal Reserve Notes that one may find in circulation, and notes not produced by the U.S. Government.