The Dashboard is what you see when you log into Umbraco:
It is very easy to add to this. Create a UserControl and perhaps watch this video http://umbraco.com/help-and-support/video-tutorials/introduction-to-umbraco/developer-introduction/using-net-user-controls first if you don't know how to do that.
Make sure the UserControl does not have a form in it with runat="server"
Next open dashboard.config in the config directory and:
And you should see your control when you refresh: