Extending Unity IDE with userland code

Is it possible for userland code to extend the Unity IDE UI?

I am interested in pinning multiple inspectors to game elements to aid in debugging.

Yes, it's possible to extend the Unity editor.

As Ehren mentioned, it is possible. A good example is AngryAnt's Behave, which implements a behavior tree creation user interface on top of Unity, or his Path that was recently released as GPL, and that extends the inspector for specific objects.

For the most part you can add UI elements to the editor, however you can't create a new 3D view (Scene View, Game View, PreviewRenderUtility such as those used in the model importer), which is a real bummer.