How can I tell iTunes to stop playing a song? I do mean stop, not pause. If one navigates to, say, the iTunes Store, the Pause button changes to Stop, but I want something that always works no matter my location in iTunes.

Workarounds (such as AppleScript) are acceptable as long as they aren't too difficult to set up.
The AppleScript for this is pretty simple:

tell application "iTunes" to stop


You can make this easily accessible by creating a Service.

Open Automator and create a new document. Select the Service document type. Set the Service receives: menu to no input. Drag in a Run AppleScript module and paste that code into its text box. Save the service and give it a name.

You'll now be able to activate it by going to the current app's menu and choosing Services-> Stop iTunes (or whatever you named the service).

You can assign a keyboard shortcut by going to System Preferences-> Keyboard-> Services, finding the service (it should be under General at the bottom) and adding a shortcut.