Featured Post Widget by tepelstreel is a customizable multiwidget, that displays a single post in the widget area. You can decide, whether or not the post thumbnail is displayed, whether the post title is above or beneath the thumbnail and a couple of more things. And of course, you can style the widget individually.
The plugin was tested up to WP 3.1 and should work with versions down to 2.7 but was never tested.
Download: http://wordpress.org/extend/plugins/post-feature-widget/