Military.com Resources

United States

Air Force Heraldry - Features a comprehensive guide to US Air Force symbols and emblems.
Ground Unit and Terrain Symbology - Displays a collection of symbols in .wmf format, to be viewed as images in MSWord.
Symbology Standards Management Committee - Definitive DOD organization for symbology management offers announcements and a collection of current documents.
US Army Heraldry - Extensive resource provides explanations and images of army emblems, flags, and insignias.

Historical

Civil War Military Insignias - See the badges worn by the Confederate and Union armies during the Civil War.
Military Organization Symbols Key - WWII resource provides a collection of US Military symbols used during the war.

NATO

NATO Symbology Guide - Gives a thorough overview of NATO's military symbols for land based systems in a pdf, Adobe Acrobat format.
Standard NATO Map Marking Symbols - Download an self-extracting Zip file of military symbols configured as a True Type font.