Google Reader Subscriber Count
Google Reader has an API that will show you how many subscribers a feed has there. This script puts that number in a little box in the bottom right corner of any page with Google Reader subscribers. For most blogs you can multiply this number by 2 or 3 and get an estimate of the total number of subscribers in all RSS readers.
You can find out all kinds of interesting things with this script. I knew, for example, that web comic XKCD was wildly popular - but I wouldn't have suspected it was as popular as it is. Now I know. (To tell the truth it was another Greasemonkey script, Google FX, that pointed me at XKCD as a related search in earlier testing!) I also know that the "featured slideshows" feed at Slideshare.net has hardly any subscribers at all - that's a shame but it makes me think I could find more hidden treasures there.
