In a comment on a previous post, Marco pointed out to a page on his site were he used Google searching to uncover Drupal sites on the web based on two criteria.

In early November 2005, he found 54,700 Drupal sites based on this search. See his Drupal.org post on this. This same search today says that there are 60,700 sites. 

Searching on "node/feed" in the URL gives 54,000. This is very close to what Marco got initially, and there should be only one page per site for this. But perhaps the "syndicate" block is not enabled by all sites?

Here are some other interesting searches, although inevitably inaccurate for various reasons: 

• Searching on "comment/reply" in the URL: 1,790,000 pages.
• Searching on "taxonomy/term" in the URL: 1,730,000 pages.
• Searching on "node/view" in the URL: 3,180,00 pages (this means older versions of Drupal. People should really upgrade!)

It would be interesting to repeat this search say six months from now and see what they return then.

If you have other tips, findings, thoughts, share them by commenting below.