Other
Flags.net
Flags.net is a database of current and historical flags for countries, political subdivisions, organizations, and the like. Flags can be searched with virtually any criteria, from the name of a country to their colors.
University of Oklahoma
University of Oklahoma College: National/historic American Flags
Presents historical and current images of state, federal and related flags including The Stars and Stripes, other United States federal flags, Flags of the Revolution, and Flags of the Confederate States.
Other
Flags of the World (Fotw)
Touts itself as the largest Internet site devoted to the study of flags.