These icons are public domain, and as such are free for any use (attribution appreciated but not required).
Note that these flags are named using the ISO 3166-1 alpha-2 country codes where appropriate.
Ruby on Rails If you are using Ruby on Rails, check famfamfam_flags_rails.
Instead of making every icon an extra request, FAMFAMFAM flag icons have been compiled into a sprite—a bunch of images in one file that uses CSS to position the images with background-position.
All icons classes are prefixed with .famfamfam-flag- for proper namespacing and scoping. This will help avoid conflicts with other tools.
See CSS file famfamfam-flags.css and sprite file famfamfam-flags.png (74 KB big).
There are 247 classes to choose from. Just add an <i> tag with the right classes and you're set.
When using beside strings of text, be sure to leave a space after the <i> tag for proper spacing.