Sage is one of the most popular feed readers simply because it integrates directly into Firefox. It’s not the most feature abundant feed reader but if you are looking for something simple to use and don’t want the extra clutter of another program running then Sage will probably be perfect.
This Firefox extension hasn’t had a update in over a year and I am still curious to whether they are working on a “major” update for the future, such as one that would constitute a version 2 branding. The latest version does finally fix the compatibility problems that it had with Firefox 2 though.
If you are using Sage then make sure you stop by their styles page so that you can customize the look of the feeds you read. I really like the default style but the “Mozilla Two Column” is definitely my favorite because of the clean-cut look.
Hopefully Mozilla recognizes how great the feed reader is that Flock has built-in and they develop something similar for Firefox 3. That is the one feature that would be great to see implemented but I don’t think I have heard any plans about this for the Firefox 3 release.