The United States Mint. https://www.usmint.gov/learn. In addition to offering for sale the United States Mint’s official products, the website provides information on the United States Mint’s history and the coins it mints.
The U.S. Government Publishing Office. https://www.govinfo.gov/#citation. GovInfo is a service provided by the Government Publishing Office that provides access to official publications from all three branches of the United States Government. A bit hard-core, but a good source for numismatic-related legislation.
The American Numismatic Association. https://www.money.org. The website provides informational resources for collectors of coins, currency, and exonumia, including online resources, diploma programs, and in-person seminars.
The Professional Coin Grading Service’s (PCGS) Coin Facts. https://www.pcgs.com/coinfacts. The website is an online encyclopedia of United States coinage.
The National Guarantee Company (NGC) Coin Explorer.https://www.ngccoin.com/coin-explorer/united-states. The website is another encyclopedia of United States coinage.
Coin Week. https://coinweek.com. The website is an online magazine whose mission is to “to inform, entertain, and educate collectors about coins, paper money, and every other area of numismatic pursuit.” Articles are available discussing many United States coins.
The Professional Coin Grading Service’s (PCGS) Photograde. https://www.pcgs.com/photograde. The website provides images of United States coin designs at various grades to help determine a coin’s approximate grade by comparing the coin to the photos that most closely match its appearance.
Greysheet. https://www.greysheet.com. In addition to providing its subscribers with information on the value of each coin, the website provides information on those coins to everyone.