Piwik for stem's site
Site analytics, most importantly website referrers, are useful to know who's citing our page and hence things that stem is being used for. Over the years I've found dozens of articles about arm this way and with it common complaints, points of confusion, etc.
We don't want to leak visitor data to a 3rd party so google analytics is out. Instead we should set up Piwik (http://piwik.org/).