If you're a frequent Google Reader user, you know full well that user comments on blog posts do not come along for the ride. On some blog feeds, it can tell you how many user comments there are, but on others, you typically have to visit the post to know.
A new solution called GReactions has attempted to fix this by slurping up comments from around the Web that are related to the post you're looking at. When it works, it's a seamless experience.
The Firefox extension is powered by Context Voice, which does the dirty work. This service tracks related conversation in places like Twitter, Digg, Reddit, WordPress blogs, and FriendFeed. It then clumps together those bits of conversation it picks up, and orders them chronologically.
To help sort through this mess, the tool lets you filter by source. You're also given a time line, which breaks down when each comment or mention is from.
In my brief testing with it installed, it was most useful with older content that had been given a chance to be passed around the Web. Newer items, especially from niche blogs, had no related discussion.
For heavy Google Reader users, this is an extension that's definitely worth installing. It doesn't actually do any of its magic until you hit the "comments" button that's added to the Google Reader interface when installed. This means it's not going to slow down the initial load of your feed, or interfere with things like Gears.
Google continues to run its own internetwork comment system on top of blog posts, which can only be seen by other Google Reader users. So short of visiting each site to see what other users are talking about, this is the next best way to quickly eyeball user discussion.