Related Posts Plugin?

I did a search for "related posts" and found that a few threads have been started in the past about the idea of creating a plugin to list related posts based on category/tag across a blogging network.

Is there something out there that exists like this and works? If not, is there an interest in developing such a plugin? I'm quite naive when it comes to this stuff but considering the fact that we have a post-indexer plugin that creates a global table, I think this would be something that would be doable.

Thoughts? Ideas?