[Update: 10/15/07 - incorporated changes by David Vrensk (and a few more from me). Now it merges in associations into the arc, and also deals with inheritance (e.g. STI).]

While googling for articles on Rails associations, I happened upon this gem of a script by Matt Biddulph. I loved it so much I made it a rake task! Once you install GraphViz like this:

sudo port install graphviz

and put dot.rake in your lib/tasks directory, then running this:

rake dot

produces diagrams like this:

And you can also import the DOT source into OmniGraffle for further editing, like this:

open -a "OmniGraffle" model.dot