This plugin is based on WP-Oldposts by Luc Saint Eli but has been written from scratch to insure maximum compatibility with WordPress 2.3.x.
When activited, OutOfDate shows a message above all posts older than the specified number of months (defaults to 12). The message, message-layout and number of months can be customized through an option-page under the Admin-section.
You can find more information here, and that's also where you can download it.