Dashboard Post-it is a simple plugin for WordPress 2.7 and higher that allows you to leave yourself or other authors a note on the dashboard. It is implemented as a configurable dashboard widget, so you can collapse it, move it around, and edit it as any other dashboard widget. It will accept plain text or (sanitized) HTML. Only users with the capability “Edit dashboard” can edit the note.
As of today, Dashboard Post-it is available for download at the WordPress plugin repository. Screenshots and installation instructions can be found over there.
I wrote the plugin for my own use. There are alternatives, but since some of them stopped working after the changes to the Dashboard in WP 2.7 and others are not widgetized, and since this is a much requested feature, I decided to submit the plugin to the repository.