www.conecaonline.org This Combined Organization of Numismatic Error Collectors of America is the largest error/variety coin club in the world, and any error or variety collector should consider joining this club. I am a member and serve on the board of directors. The club's journal "Errorscope" has useful and informational articles on error coins, and the club's attribution services are a good benefit.
www.money.org The American Numismatic Association is the largest coin club in the world, with about 25,000 members. Error collectors should join because of the many educational tools available to ANA members, including the ANA library (any error or variety publication or book, they probably have.)
Florida United Numismatists
http://www.funtopics.com - The largest coin club in Florida.
https://groups.yahoo.com/neo/groups/errorcoininformationexchange/info - A e-mail based error coin group which discussess errors and varieties.
Lincoln Cent Forum
http://www.lincolncentforum.com - Lincoln Cent Forum is a group dedicated to Lincoln cent varieties.
http://www.error-ref.com - This website gives detailed information on the various error types. The site is spearheaded by Mike Diamond, BJ Neff, Jeff Ylitalo, and Jason Cuvelier. Here is their description of the site: "Error-ref.com is a comprehensive illustrated glossary of every conceivable defect associated with the minting process. It will contain detailed illustrated descriptions of hundreds of basic error types, numerous subtypes, and assorted effects associated with them."
Patterns & Errors
http://uspatterns.com/minterrors.html - This site gives lot of information on patterns as well as some information on error coins. Check out the mint error section (the link will take you to that page.)
Franklin Half Errors
http://www.franklinlover.yolasite.com - Lots of information on Franklin half errors and more.
Coin World Archives
http://editions.amospublishing.com/wdcn/default.aspx?d=20141229 - Coin World allows non-subscribers to view Coin World Archives going about 7 years back. Click on the "Archives" button to view different issues of Coin World.
http://www.coinscan.com/index.html -- Lots of information on Canadian error coins.
http://www.vamworld.com -- Silver dollar varieties
http://varietyvista.com/index.htm - Lots of information on Varieties and the home of CONECA's listings.
http://www.smalldollars.com - Lots of information on small dollar coins (SBAs, Sacagawea's, etc.)
http://www.doubleddie.com/1801.html - Lots of great information on Wexler's Varieties.
http://errorvariety.com/index.html - Information on Lincoln cent Varieties.
http://www.coppercoins.com - Information on cent varieties.
http://lincolncentsonline.com/index.html - Information on cent varieties.
http://www.lincolncentresource.com --Lots of information on Lincoln cent varieties
http://cuds-on-coins.com - Lots of cud photos and listings, and an excellent site for information on Cuds.
VAM Silver Dollars
http://www.vamworld.com - Information on VAM Morgan and peace dollar varieties.
Misaligned Die Coins
http://www.maddieclashes.com - A site with lots of detailed information on misaligned dies.
http://www.traildies.com - Information on the "trail dies" varieties.
http://www.pcgs.com - PCGS certifies mint errors and some varieties.
http://www.ngccoin.com - NGC certifies mint errors and some varieties.
http://www.coinworld.com - Publication on all types of coins, but with information in every issue on error coins as well.
https://www.whitman.com/store - Publishers of many errors and variety books.
http://www.numismaticnews.net - Weekly publication on all topics related to coins or currency.
Other Numismatic Sites
Numismatic Crime Information Center
http://www.numismaticcrimes.org - Coin thefts and crimes are reported here. We have to keep and eye out for the "bad guys" to protect ourselves and the hobby, so subscribe to their free email alerts to stay abrest of crimes and criminals in the hobby who may show up at coin shows or your local coin shop.