It has become common practice to use CDNs for caching javascript, css, font's and more... however this does not appear to extend to icons to the same extent (possibly because each site prefers to have a unique look and feel?)
Are there widely used icon sets hosted by CDNs that are common enough to provide significant speed improvement from the CDN hosting and possibly already being pre-cached from another site using that CDN.
One example is: http://cdnjs.cloudflare.com whose contents are browse-able at https://github.com/cdnjs/cdnjs/tree/master
some icon sets it hosts are at:
/ajax/libs/twitter-bootstrap/2.3.2/img/glyphicons-halflings.png
/ajax/libs/fatcow-icons/...
/ajax/libs/foundicons/3.0.0/svgs/...
/ajax/libs/aui/5.4.0/aui/css/icons/...
/ajax/libs/topcoat-icons/0.2.0/svg/...
/ajax/libs/jqueryui/1.10.3/css/base/images/...
*jqueryui is also hosted on ajax.googleapis.com and others but is a limited set
as is the bootstrap halflings icon set:
http://netdna.bootstrapcdn.com/bootswatch/2.3.2/img/glyphicons-halflings.png
http://ajax.aspnetcdn.com/ajax/bootstrap/2.3.2/img/glyphicons-halflings.png
http://www.bootstrapcdn.com/twitter-bootstrap/2.3.1/img/glyphicons-halflings.png