Coppercoins – Doubled dies and RPMs of the Lincoln cent series are listed here with colored images. Excellent reference material. http://www.coppercoins.com/
Heritage Auctions – An excellent site for looking at great coins that have been auctioned. Great reference material. http://coins.ha.com/?ic=Tab-Home-041408
Independent Coin Graders – One of the two third party grading companies who use MADdieclashes and traildies number system on their encapsulations.
MADdieclashes.com – An excellent site dedicated to die clashes from the United States and worldwide. Abundant colored images with denominational overlays are contained in this site. http://maddieclashes.com/index.html
Traildies.com – The only site dedicated to the anomalies called trails and wavy steps.