Google Reader Subscribers Count is a discreet Greasemonkey script that shows the number of Google subscribers to a site's feeds. The number is overlayed at the bottom of each page that has feeds and it's one of the ways you can measure the popularity of a site. If you click on the number, you can subscribe to the feed.
As usually, the script requires Firefox and the Greasemonkey extension, although it could also work in other browsers. A related script is Google Reader Subscribe Button, which shows if you've subscribed to a feed and lets you subscribe to it in Google Reader. It would be interesting to combine the two scripts.