Where do I define a method so it's available to all models?

You can write an extension to put in the lib/ directory and include it
in all of your models...

module MyExtension
  def added_function

WadeWinningham is right

Yes, you can put it in the model directory, mark it as an abstract
model class, and make sure it won't collide with any other model.

You don't have to explicitly call require in other models, just
specify it as the parent class, and Rails with automatically load the
specific file. Make sure to follow the Class name -> filename
convention, so Rails can find it. Have Rails autoload it, so it can
reload the file when needed in development mode.