Error Coins

Baker Numismatics
Photographic image library of coins with doubled dies, repunched mintmarks, over mintmarks, and repunched dates.
http://www.bakercoins.net/learn/images/

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.
http://studium.com/6/errorcoins.html

Combined Organizations of Numismatic Error Collectors of America
National numismatic organization devoted to error and variety collectors.
http://conecaonline.org/

Error Coins Magazine
Monthly magazine devoted to error coin collecting. Available by subscription.
http://www.etcmmag.com/

Error World
Internet club devoted to error and variety coinage of all kinds.
http://www.surok.addr.com/

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.
http://www.geocities.com/israelerrorcoins/

Mint Error News Magazine
Available both as a print magazine and an online PDF magazine with articles, features, and discoveries of major mint errors (striking errors) from the United States and around the world.
http://minterrornews.com/

Rotated Die Coin Census
Census for US coins with major die rotation of 90 degrees to 180 degrees. Includes fake error detection and measurement indices.
http://www.rotateddies.com/

Silver Dollar VAMs
Detailed information on silver dollars, specifically Morgan and Peace Dollar die varieties known as VAMs.
http://www.robjoyce.com/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 them.
http://www.littlemistakes.com/WorldErrors/