Collectible Detective.com (scads of categories including "The Library & History Lounge > Antique & Vintage Stamps," which has links to a much broader range of sites, including postal history, than "stamps" implies).
Joe Luft's Philatelic Resources
Stamp Websites (contains links to a lot of cover web sites as well)
Stamp2.com (Formerly Francis Chan's Stamp Resources )
(Stamps only section below)
American Philatelic Research Library
British North America Philatelic Society
CedarStamps (and postal history -- in Beirut, Lebanon)
Fepowmail.com (Far East POWs)
JoeFrasketi's Space & Other Topical Covers
Japan-Japan (Includes a bourse, but also much more)
Military Postal History Society
Till Neumann Rare Stamps and Postal History (Has an extensive Links list as well)
Polish Philately (offering philatelic literature in English on many aspects of Polish postal history)
Postal History Society of China
Brad Sheff (US Postal History to 1875)
Swedish Military Postal History Society
Universal Ship Cancellation Society (focus on US Navy ships)
warcovers.com (Roger Barton, in GB)
www.Collect.at (offering a wide range of collectibles including postal history)
George Townsend Stamps (US & Worldwide. Wantlists welcome.)
I'm interested in exchanging links with other postal history sites (societies and dealers). Please e-mail me if you would like to exchange links -- and also if you find any broken links on this page.