Matt Mullenweg has released a new plugin for 2.1 and above called Random Redirects, and it does exactly what its name implies. You click on a link, and it brings you to a random post from your blog.
From the plugin information:
Allows you to create a link to yourblog.example.com/?random which will redirect someone to a random post on your blog, in a StumbleUpon-like fashion.
Already more than four hundred people have implemented the plugin, including TechCrunch.