Blog carnivals on particular topics are the blogosphere's attempt to provide authoritativeness and imprimateur (something publishers are always talking about) to blog posts. For those who haven't ridden yet, a blog carnival is a periodic "best of" collection in a certain subject area, that may have a home at a particular blog or may rotate among blogs. As you can see at indexer Blog Carnival, carnivals exist in many areas--science, education, media, politics--you name it. Many have a popular bent, but this post from Ben Vershbow at IF:Book, the blog of the Institute for the Future of the Book, lists a number of carnivals geared to scholarly communities.
To me, a carnival is a secondary publishing service, serving the same function that review journals and survey articles have for many years: making important information findable. The interesting parallel is that these are not automated selections based on number of hits or user-supplied tags like the social bookmarking sites. Posts to be included in a carnival are selected using old-fashioned editorial judgment.
Of course, they are free (or ad or donation-supported), and they have a distributed model of production, unlike the expensive operations such as Annual Reviews and the like. And so they are much less formal, and the inclusion criteria are uneven at best.
But isn't it interesting to see how new technologies keep spawning services that are so familiar?
In the blogosphere as in the traditional publishing world, there's too much out there to keep track of without finding aids. Blog Carnival itself is the Medline of the carnivals as Technorati is the PsychInfo of blogs.