When you want to learn more about the money in your wallet, the money factory is probably the first place you'd want to head. moneyfactory.com is the web site for the Treasury Department's Bureau of Engraving & Printing where your paper money comes from.
The collectors section gives topics that include fractional currency--paper money that had values between 3 and 50 cents during their times of circulation in the 1860s and 70s--as well as denominations greater than $100 that are no longer in circulation. There's also a section from the main page where you can test your knowledge of currency--from how to spot a counterfeit to games to see how much you know about your money.
For coin collectors, the US Mint's web page is the place to check out commemorative coins as well as the 50 states quarter program, including the new quarters coming this year.