ZigDashNote is a small WordPress plugin that adds a simple text widget to your Admin Dashboard page, allowing you to leave notes and reminders for yourself and fellow admins. It allows HTML if the user has the unfiltered_html capability, and will attempt to make all URLs into links.
You can change the title as well as the content, and remove the credit link if you prefer.
You can deactivate and reactivate the plugin without losing your notes.
As with all ZigPress plugins, this plugin is fully compatible with ClassicPress.