WordPress Plugin Repository has a new plugin A Year Ago Today. The plugin allows you to display Titled links to posts that were posted exactly one year ago on your WordPress site using a simple sidebar widget. All post are listed by popularity on the basis of number of comments.
Install and activate the plugin A Year Ago Today. After activation visit Dashboard -> Appearance -> Widget page and drag its widget to your sidebar or any other widget area. It allows you to use a custom widget title and number of posts to display.