Mark

Adds a simple ‘Post-it’ widget to the Dashboard that allows you to leave yourself or other authors a note. The plugin can be configured and moved around as any other Dashboard widget. It will accept plain text or (sanitized) HTML. Only users with the capability “Edit dashboard” can edit the note.
This simple plugin is based on code from <a href="http://wpengineer.com/add-wordpress-dashboard-widgets/" title="Add WordPress Dashboard widgets" rel="nofollow ugc">WP Engineer</a> and <a href="http://rick.jinlabs.com/2009/02/01/how-add-options-to-your-wordpress-27-dashboard-widgets/" title="How to add options to your WordPress 2.7 Dashboard widgets" rel="nofollow ugc">Ricardo González Castro</a>. Leave your comments <a href="http://ideophone.org/dashboard-post-it/" title="Dashboard Post-it homepage" rel="nofollow ugc">here</a>.
Warning Your notes will be replaced by boilerplate text upon upgrading. Make sure to copy the content somewhere else before you upgrade and then paste it into the new version. Sorry about that!

Dashboard Post-it

Adds a simple 'Post-it' widget to the Dashboard that allows you to leave yourself or other authors a note.

shawfactor

LH Dashboard Notes allow you to insert noes on your wp-admin dashboard for your users using the post editor interface. If this plugin is network activated the insertion and editing of these dashboard notes is centralised on the main site of your multisite install. Allowing you to set Dashboard notes that are viewed throughout your multisite network.

LH Dashboard Notes

Allows you to create and edit notes that appear on the admin dashboard

Dashboard Post-it vs LH Dashboard Notes