I've seen this used in the wild quite a few times. I'm pretty sure dome of the "universal share this button" widgets use history sniffing to put sites they know you use at the front if the list.

First? I doubt it. Also, I don't know if they were before or after these guys:

http://www.didyouwatchporn.com/

I assume this is done using the css :visited method? I saw a test page using wikipedia instead. I will post here if I find it again.

This is more certainly not the first in the wild use!

What about http://www.mikeonads.com/2008/07/13/using-your-browser-url-h... from July 13th, 2008?

Sorry, false. We used this in 2003 for commercial purposes, as a way for Site B to verify the user had used Site A without setting a cookie.

Some years ago, some people at Stanford developed the "Safe History" Firefox extension as a means of limiting IIRC history/cache sniffing via CSS. As far as I know, the extension's currency has lapsed. The idea remains valid.