WordPress' founder Matt Mullenweg just created this small plugin. http://wordpress.org/extend/plugins/random-redirect/

Its based on the idea from the latest StumbleUpon toolbar where you can Stumble random page on a certain site like CNN, YouTube and Flickr, but lets you see random pages on your blog.

It 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.