Feed Compare is essentially Alexa for Feedburner. Though the service doesn't offer a top 100, or rank each feed, it does create Alexa-style growth charts based on Feedburner subscriber numbers. The service can compare up to four feeds at once and can display between one and twenty four months of data.
The service isn't without faults, though. The biggest is that it operates understandably off of Feedburner feed URIs. For many sites, that's the same as the site name (ours is "ReadWriteWeb"), but for others, it isn't. If you enter "Problogger" into Feed Compare, for example, you won't wind up with the stats for Darren Rowse's popular blog. The correct URI for Problogger's feed is: "ProbloggerHelpingBloggersEarnMoney." The blog at Search Engine Watch is another example of this. Their URI is "sewblog."
Because many people probably don't realize that that the Feedburner URI isn't always the same as the blog name, or don't know how to find the correct URI, the service will at times offer faulty stats to people. Problogger certainly has more than 85 subscribers, but you may not realize it if you're looking at the report for the "Problogger" URI.
It would be great if Feed Compare were able to allow users to enter in URLs, then automatically discover the correct feed URI on that site, if it exists. It would also be nice if the service offered embeddable charts and even blog rankings by industry. Rankings based on Feedburner numbers might not mean much, but they could still be fun.