This is geared more to quick references than comprehensiveness.
Air Force -- http://www.af.mil/sites/alphabetical.shtml
Arlington National Cemetery -- http://www.arlingtoncemetery.org/site_map.html (Also see "funerals.")
Army -- http://www.army.mil/A-Z.htm
bases and other installations -- http://military.com/InstallationGuides/ChooseInstallation/ (Also see each service.)
Coast Guard -- http://www.uscg.mil/
decorations (medals and ribbons) -- http://foxfall.com/
Defense Department Almanac (agencies, budget, congressional committees, demographics, joint commands, leaders, ranks, statistics and top contractors) -- http://www.defenselink.mil/pubs/almanac/
equipment -- http://www.fas.org/man/index.html (Also see each service.)
foreign military -- http://www.fas.org/man/index.html
funerals -- http://www.militaryfuneralhonors.osd.mil/intro.html (Also see "Arlington National Cemetery.")
glossary -- http://call.army.mil/ (Click "thesaurus" at the Center for Army Lessons Learned. Gives infomation on each service. Some entries are only links to other entries, but listings grow quickly.)
history (capsules of U.S. conflicts) -- http://www.military.com/Resources/HistorySubmittedFileView?file=history_completelist.htm
Joint Chiefs of Staff -- http://www.dtic.mil/jcs/
legal system -- See "Uniform Code of Military Justice."
Marine Corps -- http://www.usmc.mil/marinelink/websites.nsf/unitsalpha
miscellaneous -- See "glossary," "Defense Department Almanac" and "Defense Department index."
Navy -- http://www.navy.mil/navydata/infoIndex.asp
ranks -- http://www.defenselink.mil/pubs/almanac/almanac/people/insignias/index.html
stations (as in bases and other installations) -- See "bases."
Uniform Code of Military Justice (UCMJ basics in layman's terms) -- http://usmilitary.about.com/library/weekly/aa091100a.htm
Veterans Affairs http://www.va.gov/General/site_map.htm