Ok, you're saying, so I like parties... but what are blog carnivals? It's simple, really. A group of bloggers take turns soliciting and then hosting a series of blog posts on a particular topic.
What are the benefits? Well, your readers are exposed to different opinions and writing styles, all on a topic that you choose (and that lays within the scope of the overall carnival). As a blogger, you form relationships with other people in your field, and perhaps learn something as well. You'll probably attract traffic to your blog from the authors of the various included posts.
Soooo.... why is this all coming up...? Well, a post about Google Knol from this very blog was featured in the Wild Apricot Non-Profit Technology blog carnival a few days ago. Go us! We have things to say that other people might find useful! What a concept.
Consider hosting a blog carnival yourself. Or maybe just a carnival, as long as you're not afraid of clowns.