They're about to get some competition soon from Google. GDrive ( http://www.geekwire.com/2012/google-drive-wild-screenshot-lo... ) CSS sprites are already on Google Docs:

https://docs.google.com/static/document/client/css/128155302... https://ssl.gstatic.com/docs/common/jfk_sprite40.png

.docs-icon-drive{left:-21px;top:-1460px} .docs-icon-drive-grey{left:-21px;top:-1481px} .docs-icon-drive-grey-hover{left:0;top:-1775px} .docs-icon-drive-grey-pressed{left:-21px;top:-2205px} .docs-icon-drive-hover{left:0;top:-21px} .docs-icon-drive-pressed{left:0;top:-1607px}

And the sprites in question highlighted: http://ppi.fi/gdrive.png