The flags of all countries in the world in one sprite
Include 1 css file and have all the worlds' flags on your site. Tell everyone who uses a lot of country flags to use this link, so it will be in everyone's cache!
In the head of your html file:
<link rel="stylesheet" type="text/css" href="//github.com/downloads/lafeber/world-flags-sprite/flags32.css" />
In the body of your html file:
<ul class="f32"> <li class="flag ar">Argentina</li> <li class="flag au">Australia</li> <li class="flag at">Austria</li> ... </ul>
The countries corresponding to the codes can be found at: http://en.wikipedia.org/wiki/ISO_3166-1_alpha-2
If a previously visited site uses this same code, the file is already in the cache of the user and doesn't need to be downloaded again.
See the cheese wiki: http://www.cheesewiki.com/ for an example