US Naval History and Heritage Command
The offical historical branch of the Department of the Navy has a wealth of resources online, including the Online Photo Library and a large number of documents related to Navy uniforms.
Emersoninsignia.net: US Army Insignia
Some of my most prized and valuable insignia books were written by William K. Emerson. His website is an outstanding collection of information on US Army insignia, uniforms and badges.
US Militaria Forum
A forum where a lot of very knowledgeable people discuss US militaria, this site archives images members have posted and is a wonderful reference for uniforms and insignia as they were actually used.
Quarterdeck.org: US Navy Uniforms
The San Diego Navy Historical Association has an online collection of USN uniform regulations here.
U.S. Coast Guard Uniforms & Insignia
A number of good resources on USCG uniforms, including the authoritative U.S. Coast Guard Enlisted Ratings, Rating Specialty Marks and Distinguishing Marks 1915-2011by Dana Leiwis, CWO, USCG ret.
All text and images © Justin T. Broderick, 2013 unless otherwise indicated.