A blog carnival is a really cool idea. Basically, the carnival holder, let's call them the carnival master, recruits other bloggers to write about a certain topic. After the other bloggers write their posts, the carnival master links to all of them in a post of their blog. In turn, the other bloggers link to the carnival. The carnival can be about any topic.
Participating in a carnival will boost traffic to your site and will increase your Technorati authority. Hosting a carnival will boost traffic and your Technorati authority even more.
The Carnival of the Blue is a carnival about ocean issues.
One month strong, the participants of Carnival of the Blue take turns hosting a carnival. Right now they are doing it monthly, but if it becomes more popular they will do it more often. The first one was held over on Blog Fish. My turn is on September 3.