I just hacked together a simple plugin for RoR apps in order to show a simple model diagram.
I've been a fan of RailRoad since the beginning, it generates simple dot and neato diagrams from your rails application. Now I just found out about the cool http://yuml.me beta and decided to use RailRoad to create yUML diagrams.

I created a GitHub project so you can look at the code and make whatever changes you want. The project page has more info on the plugin. I was also thinking of adding a Shoes app to create the diagrams but for now I just use it to display the diagram. Please keep in mind that the source's all messy and is far from finished,  perhaps someone else would like to make it a whole lot better ;)