A blog network is a group of blogs that are connected to each other in a network. The advantages of such networks are pretty obvious, from the SEO point of view. Fast indexing of new content, ping and trackback, one way and 3-way links, article marketing, etc... Read this article for further details and some examples: What is a blog network?
Now, finding new networks in your niche is something that you need to do it yourself. Just do a research on Google and you'll certainly find what you're looking for.
