Featured Posts is a very simple widget that allows you to select one post to display in the sidebar. Widget has interface that allows you to search for a post to display (it can be post, page or custom post type post).
Once you select the post to display, you can configure how it will look like.
This includes selecting template and style to use, and option to configure the length of excerpt. The plugin allows you to override templates in your theme and adjust the rendering to your liking.