I was at HardOCP one day and they had a news story regarding canvas fingerprinting. Here is the article they were referencing. Basically it is a way to "track" whom is using your site without using cookies. It is a sort of a spying technique and I don't like it.
http://www.propublica.org/article/meet-the-online-tracking-device-that-is-virtually-impossible-to-block
This brings me to simHQ. Last week I was tring out Tails Live DVD https://tails.boum.org/
and used it to got to simHQ website, well, Tails blocks canvas fingerprinting by sending back a blank canvas to the server that made the request and when I connected to simHQ the browser informed me that it was sending a blank canvas back to simHQ because it tried to use canvas fingerprinting.
I no longer have a membeship at simHQ and is why I posted it here instead of there. Maybe simHQ is not aware they are using addthis' or perhaps it is being used by one of their advertisers.