If you think stamp collecting is just for hobbyists and not something a shrewd investor would consider, you may want to think again. Mint condition specimens have appreciated by up to 45.5% over the past 10 years, according to analysis by StampFinder for Forbes, easily beating typical returns on real estate, gold, fine wine and the broad stock market. And the rarest philatelic treasures can sell for millions, with world record prices being set recently. Read on to see the most valuable stamps of all time.