wondermill

If you want to graphically display a random list of your posts with their featured image in the side bar, then this is the plugin for you. This plugin allows you to set the category IDs (or all categories) you want to pull the random posts from, the number of random posts to display, the maximum width and height of the featured image thumbnail, the widget title, and the colors of the surrounding box.

Random Posts with Thumbnails

Puts a list of clickable random posts with their featured image in your sidebar as a widget, this list is customizable.

mmfilesi

This plugin shows in the sidebar n post related with the same category(s) of post being viewed. 
The result can be random or chronological order.
Este plugin es un widget que se incluye en el sidebar. Muestra n entradas relacionas con la entrada que se ha cargado. 
Los resultados pueden ordenarse de forma aleatoria o por orden cronologico.
<h3>Donations</h3>
http://www.mmfilesi.com

mmfilesi-random-post

This plugin shows in the sidebar n post related with the same category(s) of post being viewed.

Random Posts with Thumbnails vs mmfilesi-random-post