Dashboard Admin Email
Dashboard Admin Email is a plugin for WordPress that displays the administrator's email on the dashboard. It's not rocket science, but it's a time saver for admins who manage WordPress blogs for clients. I can confirm it works with WordPress 2.9.2 and 3.0.
Dashboard Message is a WordPress plugin that displays a custom message on the Dashboard. The plugin will display anything you throw at it, which can be useful for messages to users or quick-and-dirty plugins. I can confirm the plugin works with WordPress 3.0
Using the plugin is very simple: put whatever you want to display in the message.php file in the plugin's folder. If you need any help, there is the comments section below.