Links

Here are some links to other philatelic sites I find interesting and useful, with some annotations.
Society Websites
American Philatelic Society:  The national stamp collecting organization.  If you're a collector, you should belong to this organization.  Includes dealer listings, club listings, new collector resources, and much more.
American Association of Philatelic Exhibitors:  If you are an exhibitor, or want to be one, join this organization.
United States Philatelic Classics Society:  Focus on 19th century US.  Tremendous resources online, including digitized books such as the American Stampless Cover Catalog.
United States Stamp Society:  US specialty society with focus on the bureau issue of 1894 and later.  Excellent for collectors of 20th century material.
Ohio Postal History Society:  I'm biased, I'm the president.  If you collect Ohio, this is the Society for you.
Machine Cancel Society:  If you are interested in postal markings by machine, take a look. 
Mobile Post Office Society:  If you are interested in RPO, railroad, streetcar, or other transit based postal markings.
American Airmail Society:  The premier society for those interested in airmail.
American Revenue Association:  All things in US revenues.
American Stamp Dealers Association:  Dealer listings, resources on buying and selling, and a particular tip of the hat to their monthly magazine.  You don't need to be a dealer to subscribe.
Youth and New Collectors
APS Young Collector Resources:  Various activities and materials, notably including a link to information about Young Stamp Collectors of America, a youth stamp club organized by APS.
Stamp Collecting for Kids and Adults Alike:  short article and link aggregation with lots of good resources for collectors.
Starting a Collection:  It's easy to start a stamp collection - just watch for stamps on your incoming mail.  But this APS link has some additional thoughts and resources. 
Research and Resources
US Postal Bulletins:  Original source for post office openings and closings, postal regulation changes, and much more.  Also has searchable Postal Laws & Regulations for those seeking a deeper understanding of mail handling.
US Post Office Listings:  Created by dealer Jim Forte, this is a searchable list of PO names with year dates of operation and counties.  Particularly useful for the ability to search partial names. 
USPS Postmaster Finder:  USPS research on historical names of postmasters.  Highly incomplete but sometimes will return impressive detail.
Doane Cancels:  Listings of Doane cancels from all 50 states with reported periods of use, along with other information.
North Carolina Postal History Society Catalog:  Remarkable listing of NC postal markings, frequently updated.
National Postal Museum Research Links:  In particular, the NPM links page lists most state postal history societies.
Philatelic Forums
Philamercury:  Hosted by Richard Frajola.  High level postal history discussion.
Delphi Stamp Forum:  Long-lived forum, with particular interest in first day covers.
Stamp Collecting Family:  Large and vibrant forum covering a wide variety of US and international topics.
Stamp Collecting Spot:  New forum with additional social media capabilities.  Worth a look.