Visualize Models is a small rake script that will generate .png images for RoR models (i.e. the database tables) that will display the table/column information. The associations between tables is based on the default RoR naming convention (i.e. <table>_id columns). See image below for the typo blog models (click to enlarge):

Install as a plugin. From your rails application root, run

 > ruby script/plugin install svn://rubyforge.org//var/svn/visualizemodels/visualize_models 

The above may fail if your rails application is under svn control, in this case you'll need to add "-x" parameter.

 > ruby script/plugin install -x svn://rubyforge.org//var/svn/visualizemodels/visualize_models

Run the plugin with:

 > rake visualize_models

To remove the plugin, run

 > ruby script/plugin remove svn://rubyforge.org//var/svn/visualizemodels/visualize_models 

This plugin depends on GraphViz, which you can find here.

If you're a Mac user, the Darwin port of GrapViz seems to work better. Just do:

 > sudo port install graphviz

This program has its roots in the Annotate Models by Dave Thomas. See also the rubyforge project page for additional info.