Pete Loeser’s website Historical Flags of Our Ancestors has grown over time into a wonderful resource for vexillologists and flag enthusiasts. It includes over 100 pages and over 3000 images, with a lot of original content (for example, see the “Short Papers and Vexillological Charts” section).
The main site can be found at www.loeser.us/flags. In addition, the PFA is happy to host a copy (mirror) of the site at flags.mainzone.com. Enjoy!