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     "stuff"   end end

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.