In the past, most plugins that have been made were project specific. WP Notes Widget is the first publicly available plugin which can be downloaded from the WordPress plugin repository. You can check it out at https://wordpress.org/plugins/wp-notes-widget/. Here is the description taken from the plugin page:
Posts and pages have their own characteristics and uses, but sometimes there is a need to display important, very short, time sensitive information which don’t really fit into a post or page. WP Notes Widget fills this gap. The visual design is similar to real sticky notes which adds to the effective communication of the message.
WP Notes Widget is built with WordPress Plugin Boilerplate.