Karl Horak, A.K.A. Schlepp, has started an initiative that he calls the Great Backyard Plone Count - GBPC. Modeled after the Audubon Society's Great Backyard Bird Count, the GBPC attempts to survey Plone sites in the wild. The general idea is that everyone in the Plone Community spends time each day during the sample period looking for sites running Plone. I am a little to late to this as the dates for the count are February 13-16, 2009 so you will have to participate next year. One thing that I think could improve the GBPC support materials is a listing of what to look for when trying to identify a Plone site.
As a web content management consultant, I have a keen interest in knowing the CMS behind the site. In fact, I have a hard time visiting a site without trying to guess. Over the years, I have developed an awareness of the various "tells" associated with the different CMS platforms. I also like to keep a list of known sightings such as when Dries Buytaert posts announcements of new Drupal sitest. I got all excited when Deane Barker told me about the Backend Info Plugin for Firefox which is supposed to figure out what is running the site you are on. What a letdown when I learned how few technologies it knows about! It didn't even catch obvious tells like the commas in the URLs of old Vignette Story Server sites or the word "Satellite" that appears in the URLs of so many FatWire sites. It didn't even catch sites with "generator" meta tags.
Since reading about the GBPC, I never realized how much like bird watching my "hobby" of CMS identification is (only probably less cool). Does anyone else have this crazy hobby?
[Edit] The results are in. People submitted 183 sites over the 96 hour collection period. It looks like around 40 people participated. Pretty good for a first go around. I would expect with some more publicity, this could initiative could return much more results. Karl did a sweep of Delicious links and found 2,326 tag with plone-site. Of course, the thing that this method has over Delicious is that it gets to the internal sites running Plone.