The Transport Politic is one of the many wonky transportation/policy blogs I read. As I was making a comment on one of their stories, I was pleasantly surprised by their comment preview system.
It shows the results of your post, formatting and all, in real time right below where you're writing. Pretty slick, huh? If I might make a suggestion, though, I'd recommend putting them side-by-side instead in case someone is writing a novel in the comments (people do that). If the preview for the part of the comment you're writing falls off the page, it's no longer useful.