Four Nodes

Simple plugin to display random posts on your wordpress blog. Plugin provides a widget to display random posts in sidebar. 
Also comes with a shortcode [randomposts] to include on pages.
<ul>
<li>Easy to install</li>
<li>Picks Random Posts</li>
<li>Ability to select from specific category</li>
<li>Limit number of posts</li>
<li>Shortcode support</li>
</ul>

FourNodes Random Post

Displays Random wordpress posts in a sidebar widget. Also, provides shortcode to display random posts on pages.

Mark (podz)

Displays random psots on the sidebar of your blog, ideal for keeping visitors on your site longer and offering more reading.
This widget is also ideal for SEO as it increases the number of pages interlinked on your site, helping the search engine spiders crawl your site more efficiently.
Features:
<ul>
<li>configurable number of displayed records;</li>
<li>conclusion quote, or just the title;</li>
<li>display in a list or any other form (the tags are set).</li>
</ul>

RandomPosts Widget

Displays random psots on the sidebar of your blog, ideal for keeping visitors on your site longer and offering more reading.

FourNodes Random Post vs RandomPosts Widget