Richard McManus is digging into market share for RSS aggregators, aka newsreaders:
He writes: "But there's been little talk of who is winning the battle for the eyeballs of RSS consumers. Mainly that's because reading RSS feeds is still a niche activity, but who's to say that 2005 won't be the breakthrough year for RSS Reader software?"
Checking his own Feedburner stats, Richard says he gets the following data:

Richard points out these stats can be squewed because they don't reflect active accounts; web-based and downloadable tools track differently, and sites ping blogs on different schedules to pick up feeds.
In the comments Noah Brier references an article he wrote on this in the fall. At the time, he found that the most-used list(also via Feedburner) started with Bloglines as well.
(Via Winged Pig)