The 1847 issue
A comprehensive article by Calvert Hahn
The 1851 3¢ issue
A comprehensive study
The 1851 1¢ and 3¢ issues
Just unbelievable. A huge, huge study on these two stamps.
The 1851 3¢ issue
Great guide to this issue, well laid out site
>The 1869 Pictorials
A comprehensive article on this issue by the great man himself, the recently departed Calvert Hahn
 The 1869 90¢
A development of this popular Lincoln.
The 1903 1¢
A complete study liberally sprinkled with quality images.
The 1875 1¢
The definitive site on the 1¢ Franklin issue. The site is working towards a pictorial description of every plate position.
The 1903 issue
An informative site that contains a brief overview of each of the values and their uses.
The Museum of Essays and Proofs
Gradually expanding site that had large clear images. Beautiful site