Even though stamp collectors have been actively engaged in their hobby since the advent of stamps, they have not always been happy with their choices.

Many of them have been very vocal about the U.S. Postal Service's decision to launch a Looney Tunes cartoon series, which began with Bugs Bunny and continued with such potential blockbusters as Tweety Bird and Sylvester the Cat.There will be 15 characters, although some collectors feel they trivialize what many regard as a sacred tradition. A commercial cartoon character, they say, violates the Postal Service's own rules against depicting commercial products on stamps.

The first commemorative stamps were issued in 1893 to honor the World Columbian Exposition held in Chicago. The stamps celebrated the 400th anniversary of Christopher Columbus' voyage to the New World.

Even though the stamps were highly popular with collectors and customers - 2 billion stamps were sold at the Columbian Exposition for a total of $40 million - a number of critics denounced them. The designs were based on paintings by different artists, each of whom had a different impression of Columbus.

The 1-cent stamp showed Columbus with a clean-shaven face, spying land from his ship, while the 2-cent showed him landing a few hours later with a full beard.

There were problems with the denominations, too, suggesting that the $2, $3, $4 and $5 Columbian stamps were useless for mailing.

The postmaster general who backed the stamps, John Wanamaker, spent $10,000 of his own money to buy 5,000 of the $2 stamps, then put them in his safe. When he died in 1926, the stamps were valued at $4.50 each.

Commemorative stamps have been proliferating ever since.

We just celebrated the 100th anniversary of the Spanish-American War and the sinking of the U.S. warship Maine in Havana Harbor on Feb. 15, 1898. The stamp features the USS Maine under power with the slogan "Remember the Maine" above and the United States of America below.

February also kicked off the "Celebrate the Century" series, with the Postal Service issuing two sheets of 15 stamps each - one honoring the 1900s and the other honoring the 1910s.

For the next two years, stamps will be issued to honor the most memorable people, places, events and trends of each decade of the 20th century.

For the first time, Americans are being invited to put their own stamp on history by voting on subjects that will be featured on stamps honoring the 1950s. Official ballots for stamp subjects are available at all post offices, at a dedicated Web site (http://stampvote.msn.com) and in some national magazines.

The "Celebrate the Century" series features such subjects as the arrival of immigrants at Ellis Island, black scholar W.E.B. DuBois, the Model T Ford, the teddy bear, the first baseball World Series, the Gibson girl, Crayola Crayons, the Grand Canyon, the Panama Canal and the first crossword puzzle.

On March 19, the Postal Service issued the "Five Flowering Trees" commemorative stamps to honor the Southern magnolia, the blue paloverde, the yellow poplar, the prairie crab apple and the Pacific dogwood. On March 25, postal authorities introduced five stamps honoring the genius of American sculptor Alexander Calder.

In the "Great Americans series," the Postal Service has already honored Thomas Jefferson, Harry Truman, Jack London, Margaret Mitchell, Buffalo Bill Cody and Sitting Bull.

On April 3 the service will add Henry Luce, the founder of Time magazine, to the list, for "his unmatched devotion to journalistic excellence."

If you're interested in finding more information about the U.S. Postal Service, the Web is a good place to go. The main site is (www.usps.gov), and it branches off in a dozen directions. For frequently asked questions, check out (www.usps.gov/fyi/welcome.

htm). A 12-point list outlining the Citizens' Stamp Advisory Committee criteria for the selection of stamp subjects can be found at (www.usps.gov/websites/depart/stamps/stampsel.htm).