I'd like to give unregistered visitors the ability to submit articles. Any way to do this? Ideally, there would be some sort of approval process (ideally, ideally, by email)

Now this plugin is something for me :)

Let me know if you find a solution for this!

I ended up creating a form outside of K2 and appending to jos_k2_data. Not an ideal solution though, since I still have to log in to publish the article. I was hoping I could approve by clicking a link in the email, but that's above my pay grade - which is $0 when it comes to PHP/SQL programming.