I just noticed last week that some of the online web aggregators (such as bloglines, newsgator online, live journal, etc) will send the number of subscribers in their HTTP user agent. For example:

NewsGatorOnline/2.0_(http://www.newsgator.com;_20_subscribers)
Bloglines/2.0_(http://www.bloglines.com;_81_subscribers)
ThePort_Web/1.0;_subscribers_1
LiveJournal.com_(webmaster@livejournal.com;_for_your_url/;_1_readers)

Note that live journal is using _num_readers instead of _num_subscribers. There is no standard for this as far as I know, but it would be nice if they all played together nice.

This is really handy for a site like dealazon that lets you create RSS feeds dynamically (there are hundreds of different RSS feeds currently being served). Using these user agents I can get an estimate of how many subscribers there are.