Quite a few UGC sites are allowing users to view a thread/topic, create a reply and sign up all at once. It's a very efficient and effective way of encouraging guest users to post and become a member. For example, a thread would show the editor at the bottom of the page as usual, but when clicking "reply" you'd be prompted to choose a username and password. Very quick and painless, but it's very effective in encouraging users to engage. I'm not aware of a way to do this at the moment (core or add-on), but if I'm wrong then please do let me know - as I would very much like to implement this on my site.