It’s a fairly simple extension with no options, and only adds a single button to the bottom toolbar on Google Reader’s feed items. Hit “Show comments,” and you’ll see at a glance what folks are posting and discussing in relation to the main link from the RSS item. It doesn’t show these comments by default, which is a pretty great thing, and while it would be great to have a sorting option or two other than reverse chronological, gReactions delivers what it promises.

From a LifeHacker article about GReactions