More than 220 free flag icons representing countries and unions all over the world in PNG format. Sizes : 16×16, 24×24, 32×32, 48×48.
How to change icons in Mac OS X: Copy icon (Cmd+C). Select Application. Right Click-Get Info (Cmd+i). Select application icon in top left corner of window. Paste new icon (Cmd+V) To revert default icon: Select icon in the top left corner of info window and delete it (backspace)
Just wanted to add this flag set to the list: http://www.nordicfactory.com/creative/illustrations/download-free-flag-set-png/
It has 230 flags in PNG, 256x256px and a freebie.