Baker Numismatics - Photographic image library of coins with doubled dies, repunched mintmarks, over mintmarks, and repunched dates.
Canadian Error Coins - Information and some images of circulating errors, including an overview of die varieties. Site in English and French.
Collecting Error Coins - Just as nearly every family has its black sheep, virtually every coin series has its failures. These are known as errors or mis-struck coins, and have a following all their own.
Combined Organizations of Numismatic Error Collectors of America - National numismatic organization devoted to error and variety collectors.
Error Coins Magazine - Monthly magazine devoted to error coin collecting. Available by subscription.
Error World - Internet club devoted to error and variety coinage of all kinds.
Israel Error Coins - The site focuses on Israeli error coins but also features Canadian, U.S. and others. See bi-metallic errors, coins struck on scrap metal, and others.
Jason's Error Coins - Images of a collection of error coins. Wheat cents, Mercury dimes and other assorted.
Rotated Die Coin Census - Census for US coins with major die rotation of 90 degrees to 180 degrees, includes Fake error detection and measurment indices.
Silver Dollar VAMs - Detailed information on silver dollars, specifically Morgan and Peace Dollar die varieties known as VAMs.
World Error Coins - Describes and illustrates dramatically mis-struck foreign coins. Includes information on recognizing various types of error coins as well as articles on how to get started collecting eror them.