WEBrick is an HTTP server built that is part of Ruby 1.8. It is written in Ruby, and features an easily extendable servlet system and handles directory indexes, CGI, ERb, and simple ruby proc servlets out of the box.
Gnome's Guide to WEBrick is the best WEBrick guide available. Highly recommended.
You can find a few examples of how to use WEBrick at WEBrick's home page. I've also written the following article that provides more detail on how to write servlets for WEBrick.