Open post plugin allows anonymous unregistered users to comment on selected posts, when you have a register to comment blog.
The "Open Post" plugin for WordPress was written for websites and blogs that are powered by WordPress, that choose to enable the commenting feature to registered users only. However, sometimes these owners might want to allow anonymous users to post comments on selected posts and pages. This is where the simple and basic Open Post plugin comes into play. On a website or blog where comments can only be posted by registered users, you can use this plugin to override this on posts and pages of your choice.
Check out some of the other WordPress plugins by the same author.