Lots and lots of flags
This is a big repository of flag images for countries (and subdivisions). The flags are all named according to ISO 3166 alpha-2 for easy use in applications. You can download the entire repository, or you can access individual flags via http://flags.ox3.in (see below for details).
You can see a list of all the flags available and the names/codes/aliases for the corresponding countries in this spreadsheet.
You can get the flag for any country just by its two-letter code. Just access http://flags.ox3.in/svg/xx.svg, where "xx" is the ISO 3166 alpha-2 country code (in lowercase).
This repository also has flags for many subdivisions of countries (including U.S. states, Canadian provinces, etc). Just access http://flags.ox3.in/svg/xx/yy.svg, where "xx" is the first part of the ISO 3166-2 code and "yy" is the second part after the dash (again, both parts in lowercase).
I have also added flags for many U.S. cities. There is no proper code standard I know of for identifying cities, so the format I used is simply http://flags.ox3.in/svg/us/xx/yy.svg, where "xx" is the two-letter state abbreviation in lowercase, and "yy" is the name of the city. The city name should be all lowercase, with spaces changed to underscores and all other non-alphabetic characters removed (i.e. "St. Paul" becomes "st_paul").
If SVGs don't suit your purpose, you can also access very low resolution PNGs via http://flags.ox3.in/mini/xx.png and http://flags.ox3.in/mini/xx/yy.png. However, be warned that I will probably be changing the directory structure for PNGs in the near future, so you shouldn't rely on it.
I originally made this repository to allow me to easily embed flags in Google Sheets. You can read more about how to do that here.
Yes! Most of these flags are public domain images from Wikimedia Commons, so you can do with them what you want. Flags that are not freely licensed are listed in the SPECIAL_LICENSES.csv (note that the special licenses are only for the SVG recreations of the flags; the flags themselves are still public domain). Everything else in this repository that is not an image file is licensed under the MIT license.
