Sometimes it is necessary for me to visualize my Models and Controllers to match them against some hand drawn wireframes to see the progress and gaps. Although it's handy to have everything visualized to see whats going on under the hood of legacy applications and more important models/schemas. I found a wounderful gem which provides this functionality using graphviz called RailRoad.

There are also some alternative versions you should check out:
ddollar - railroad - Thanks, Bruno Michel
royw - railroad_xing - Support for Merb & DataMapper