Taxis is a simple Rails 3 plugin that will give you a taxonomy structure for your rails application. It was written with the intent of providing a category tree for e-commerce solutions. It is in early development right now so please feel free to submit issues and provide any feedback.
gem install taxis rails g taxis:install
Taxis is heavily based off the taxonomy system originally written by Sonny Cook as currently implemented in Spree (http://spreecommerce.com/). While much of the views are the same, the admin controllers and models have been written from the ground up to isolate functionality only needed for the gem.
Fork, modify, send me a pull request