EditorWindowExample from the Unity Asia Bootcamp 12 talk "Streamlining your Unity editor". A simple node based editor.

EditorWindowExample.cs

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.Collections.Generic;


public class EditorWindowExample : EditorWindow
{
	List<Node> nodes = new List<Node> ();


	[MenuItem ("Window/EditorWindow example")]
	static void Launch ()
	{
		GetWindow<EditorWindowExample> ().title = "Example";
	}


	void OnGUI ()
	{
		GUILayout.Label ("This is an editor window - the base of any completely custom GUI work.", EditorStyles.wordWrappedMiniLabel);

		// Render all connections first //

		if (Event.current.type == EventType.repaint)
		{
			foreach (Node node in nodes)
			{
				foreach (Node target in node.Targets)
				{
					Node.DrawConnection (node.Position, target.Position);
				}
			}
		}

		GUI.changed = false;

		foreach (Node node in nodes)
		// Handle all nodes
		{
			node.OnGUI ();
		}

		wantsMouseMove = Node.Selection != null;
			// If we have a selection, we're doing an operation which requires an update each mouse move

		switch (Event.current.type)
		{
			case EventType.mouseUp:
			// If we had a mouse up event which was not handled by the nodes, clear our selection
				Node.Selection = null;
				Event.current.Use ();
			break;
			case EventType.mouseDown:
				if (Event.current.clickCount == 2)
				// If we double-click and no node handles the event, create a new node there
				{
					Node.Selection = new Node ("Node " + nodes.Count, Event.current.mousePosition);
					nodes.Add (Node.Selection);
					Event.current.Use ();
				}
			break;
		}

		if (GUI.changed)
		// Repaint if we changed anything
		{
			Repaint ();
		}
	}
}

Node.cs

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;


public class Node
{
	const float kNodeSize = 50.0f;

	static Node selection = null;
	static bool connecting = false;

	Vector2 position;
	Rect nodeRect;
	string name;
	List<Node> targets = new List<Node> ();


	public Node (string name, Vector2 position)
	{
		this.name = name;
		Position = position;
	}


	public static Node Selection
	{
		get
		{
			return selection;
		}
		set
		{
			selection = value;
			if (selection == null)
			{
				connecting = false;
			}
		}
	}


	public Vector2 Position
	{
		get
		{
			return position;
		}
		set
		{
			position = value;

			nodeRect = new Rect (
				position.x - kNodeSize * 0.5f,
				position.y - kNodeSize * 0.5f,
				kNodeSize,
				kNodeSize
			);
		}
	}


	public ReadOnlyCollection<Node> Targets
	{
		get
		{
			return targets.AsReadOnly ();
		}
	}


	public void ConnectTo (Node target)
	{
		if (targets.Contains (target))
		{
			return;
		}

		targets.Add (target);
	}


	public void OnGUI ()
	{
		switch (Event.current.type)
		{
			case EventType.mouseDown:
				if (nodeRect.Contains (Event.current.mousePosition))
				// Select this node if we clicked it
				{
					selection = this;

					if (Event.current.clickCount == 2)
					// If we double-clicked it, enter connect mode
					{
						connecting = true;
					}

					Event.current.Use ();
				}
			break;
			case EventType.mouseUp:
				// If we released the mouse button...
				if (selection == null)
				// ... with no active selection, ignore the event
				{
					break;
				}
				else if (selection == this)
				// ... while this node was active selection...
				{
					if (!connecting)
					// ... and we were not in connect mode, clear the selection
					{
						Selection = null;
						Event.current.Use ();
					}
				}
				else if (connecting && nodeRect.Contains (Event.current.mousePosition))
				// ... over this component while in connect mode, connect selection to this node and clear selection
				{
					selection.ConnectTo (this);
					Selection = null;
					Event.current.Use ();
				}
			break;
			case EventType.mouseDrag:
				if (selection == this)
				// If doing a mouse drag with this component selected...
				{
					if (connecting)
					// ... and in connect mode, just use the event as we'll be painting the new connection
					{
						Event.current.Use ();
					}
					else
					// ... and not in connect mode, drag the component
					{
						Position += Event.current.delta;
						Event.current.Use ();
					}
				}
			break;
			case EventType.repaint:
				GUI.skin.box.Draw (nodeRect, new GUIContent (name), false, false, false, false);
					// The component box

				if (selection == this && connecting)
				// The new connection
				{
					GUI.color = Color.red;
					DrawConnection (position, Event.current.mousePosition);
					GUI.color = Color.white;
				}
			break;
		}
	}


	public static void DrawConnection (Vector2 from, Vector2 to)
	// Render a node connection between the two given points
	{
		bool left = from.x > to.x;

		Handles.DrawBezier(
			new Vector3 (from.x + (left ? -kNodeSize : kNodeSize) * 0.5f, from.y, 0.0f),
			new Vector3 (to.x + (left ? kNodeSize : -kNodeSize) * 0.5f, to.y, 0.0f),
			new Vector3 (from.x, from.y, 0.0f) + Vector3.right * 50.0f * (left ? -1.0f : 1.0f),
			new Vector3 (to.x, to.y, 0.0f) + Vector3.right * 50.0f * (left ? 1.0f : -1.0f),
			GUI.color,
			null,
			2.0f
		);
	}
}