I found this amongst the entries for the WordPress Plugin Contest, which generally inspires some good work. There are some other entries which might find a home here or even on Tiggercam, but this one stood out as something that’s of immediate use.
I quite often come back to posts to add updated information, or corrections to things that have changed, or indeed anything else. I generally do that by editing the post, adding a <blockquote> and typing the extra information. But this plugin makes it simpler: it adds a box to the post editing page in which you can type your updated information. Save the changes and it adds a box to the post for you. It comes with a few preset colour schemes for the box, and you have the option of changing the HTML it uses in its options page.
I’ve given it a quick try on a few posts, and I like what it does, so I’ll be using it in future.