Collectible coins include various types such as commemorative coins, proof coins, and bullion coins, each often sought after by numismatists for their rarity, condition, and historical significance. The United States Mint produces different denominations, including the penny (1 cent), which can be rolled in quantities for collectors. The 1999 Denver mint marks a significant year for the Lincoln penny, as it represents the final year of the coin's design before the introduction of the state quarters. Collectors value coins based on factors like mint mark, condition, and rarity, influencing their market price and desirability.