Easiest way to read in a configuration file at run-time

Is there a build-in method to read some sort of 'config' file for a Unity3D application, or do I have to use the Mono's file-reading classes?

PlayerPrefs is your best friend.

You can use TextAsset files to get easy access to text

Drag any .txt file into the project and add it into your scene/resources/etc, loading it as you would a texture or any other unity asset

It'll embed your text into the application, and is easily accessible with yourTextAsset.text or yourTextAsset.bytes

They are, however, read only, so they're more useful for things which don't need to be saved back