Charles J. Di Como's Site, Featuring the U.S. 1851-61 1-Cent and 3-Cent Stamps
Many thanks to Charles for providing vital research and images for this site.

Roy Weber's U.S. 1851-57 3-Cent Stamp Postal History Site

Steven Ruecker's Image Archive of Plated 1851-57 3-cent Imperforates

The U.S. Philatelic Classics Society

The American Philatelic Society