While there are currently more than 400,000 different types of stamps in circulation the world over, old stamps--those dating from the early 20th century and before--can be worth hundreds of thousands of dollars. Philatelists catalog and review old stamps regularly. One of the most trustworthy guides is the Scott Catalog, which documents many critically important U.S. stamps, including the newspaper stamps which evolved immediately following the conclusion of the Civil War.

International interest in old stamps abounds. The Philatelic Society of New Zealand, which was founded in 1888, maintains a vast library of unique vintage stamps. The earliest universal one-penny stamps were issued in Britain and the Australian colonies during the 1830s. Just to get an idea of how much these old stamps are worth, consider that a one-cent stamp from New South Wales printed in 1850 is now worth more than $5,000 in today's money.

Similarly, stamps dating back to Australia's first national printing in 1913 are worth around $4,000 each in mint condition. In the late 1900s, the Postal Service experimented with a program that allowed companies to advertise to customers on the backs of three-cent U.S. postal stamps. The Chinese produced what was largely considered to be the world's largest stamp, at 210 by 65 mm, during the early 20th century. These unique stamps are highly collectible, and their resale value reflects this.

Of course, stamps don't have to be old to be interesting or to have value. In 1969, the Apollo 11 astronauts brought back an imprint of the moon to create a 10-cent stamp release, which is now highly collectible. Similarly, governments in the tiny countries of Tonga in the Pacific and Bhutan in the Himalayas have produced stamps imitating fruits and records, respectively, which have also become popular among certain collectors. Thus, historical caché is not the only measure of a stamp's value. As a collector, you can focus on theme or location to build a great collection.