Feedburner
From DaveWiki
Feedburner is a Google service which enables you to track the number of people who are subscribed to your site's RSS feed.
It tells you information such as how many people access your feed, what software they use to do so, and how many people download enclosures from the feed (such as audio files in the case of a podcast).
You can also use Feedburner to set up an email subscription to your feed - so users can get content to their inbox rather than their aggregator. Again, you get to see stats on how many people do this.
Further reading
