Back in the day, paper U.S. Treasury bonds were a popular gift for bar mitzvahs, birthdays and other gift-giving occasions. These savings bonds were great because the buyer only paid half the face value of the bond (e.g. $50 for a $100 bond) and when they reached maturity, they could be cashed in for their full face value. If the bondholder waited even longer, the bonds would continue to accrue interest and be worth even more.
You can still buy savings bonds today, but they are in electronic form and you must pay the full face value of the bond. Learn more at Treasury Direct.
One reason the federal government switched to electronic savings bonds is that so many people lost their paper bonds. To help people claim lost bonds and other Treasury securities, the government launched Treasury Hunt, a searchable database of unclaimed bonds. Just enter your Social Security number to see if you have any securities on file. Warning: While the searching will be quick, the procedure to actually get your money will take a while. (This is the government, after all).
While you're at it, you might want to see if the government is holding any unclaimed income tax refunds for you. The IRS says that millions of dollars in refunds are returned every year as undeliverable [source: IRS]. Visit the unclaimed money section of USA.gov for more information.