Example code for "GUI drag drop" blog post on AngryAnt.com

DataObject.cs

using UnityEngine;
using System.Collections;

public class DataObject : GUIDraggableObject
// This class just has the capability of being dragged in GUI - it could be any type of generic data class
{
	private string m_Name;
	private int m_Value;

	public DataObject (string name, int value, Vector2 position) : base (position)
	{
		m_Name = name;
		m_Value = value;
	}

	public void OnGUI ()
	{
		Rect drawRect = new Rect (m_Position.x, m_Position.y, 100.0f, 100.0f), dragRect;

		GUILayout.BeginArea (drawRect, GUI.skin.GetStyle ("Box"));
			GUILayout.Label (m_Name, GUI.skin.GetStyle ("Box"), GUILayout.ExpandWidth (true));

			dragRect = GUILayoutUtility.GetLastRect ();
			dragRect = new Rect (dragRect.x + m_Position.x, dragRect.y + m_Position.y, dragRect.width, dragRect.height);

			if (Dragging)
			{
				GUILayout.Label ("Wooo...");
			}
			else if (GUILayout.Button ("Yes!"))
			{
				Debug.Log ("Yes. It is " + m_Value + "!");
			}
		GUILayout.EndArea ();

		Drag (dragRect);
	}
}

GUIDraggableObject.cs

using UnityEngine;
using System.Collections;

public class GUIDraggableObject
{
	protected Vector2 m_Position;
	private Vector2 m_DragStart;
	private bool m_Dragging;

	public GUIDraggableObject (Vector2 position)
	{
		m_Position = position;
	}

	public bool Dragging
	{
		get
		{
			return m_Dragging;
		}
	}

	public Vector2 Position
	{
		get
		{
			return m_Position;
		}

		set
		{
			m_Position = value;
		}
	}

	public void Drag (Rect draggingRect)
	{
		if (Event.current.type == EventType.MouseUp)
		{
			m_Dragging = false;
		}
		else if (Event.current.type == EventType.MouseDown && draggingRect.Contains (Event.current.mousePosition))
		{
			m_Dragging = true;
			m_DragStart = Event.current.mousePosition - m_Position;
			Event.current.Use();
		}

		if (m_Dragging)
		{
			m_Position = Event.current.mousePosition - m_DragStart;
		}
	}
}

MyEditorWindow.cs

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

public class MyEditorWindow : EditorWindow
{
	private List< DataObject > m_Data = new List< DataObject > ();
	private bool doRepaint = false;
	private Rect dropTargetRect = new Rect (10.0f, 10.0f, 30.0f, 30.0f);

	public MyEditorWindow ()
	{
		m_Data.Add (new DataObject ("One", 1, new Vector2 (20.0f * Random.Range (1.0f, 10.0f), 20.0f * Random.Range (1.0f, 10.0f))));
		m_Data.Add (new DataObject ("Two", 2, new Vector2 (20.0f * Random.Range (1.0f, 10.0f), 20.0f * Random.Range (1.0f, 10.0f))));
		m_Data.Add (new DataObject ("Three", 3, new Vector2 (20.0f * Random.Range (1.0f, 10.0f), 20.0f * Random.Range (1.0f, 10.0f))));
		m_Data.Add (new DataObject ("Four", 4, new Vector2 (20.0f * Random.Range (1.0f, 10.0f), 20.0f * Random.Range (1.0f, 10.0f))));
		m_Data.Add (new DataObject ("Five", 5, new Vector2 (20.0f * Random.Range (1.0f, 10.0f), 20.0f * Random.Range (1.0f, 10.0f))));
	}

	[MenuItem ("Window/MyEditorWindow")]
	public static void Launch ()
	{
		GetWindow (typeof (MyEditorWindow)).Show ();
	}

	public void Update ()
	{
		if (doRepaint)
		{
			Repaint ();
		}
	}

	public void OnGUI ()
	{
		DataObject toFront, dropDead;
		bool previousState, flipRepaint;
		Color color;

		GUI.Box(dropTargetRect, "Die");

		toFront = dropDead = null;
		doRepaint = false;
		flipRepaint = false;
		foreach (DataObject data in m_Data)
		{
			previousState = data.Dragging;

			color = GUI.color;

			if (previousState)
			{
				GUI.color = dropTargetRect.Contains (Event.current.mousePosition) ? Color.red : color;
			}

			data.OnGUI ();

			GUI.color = color;

			if (data.Dragging)
			{
				doRepaint = true;

				if (m_Data.IndexOf (data) != m_Data.Count - 1)
				{
					toFront = data;
				}
			}
			else if (previousState)
			{
				flipRepaint = true;

				if (dropTargetRect.Contains (Event.current.mousePosition))
				{
					dropDead = data;
				}
			}
		}

		if (toFront != null)
		// Move an object to front if needed
		{
			m_Data.Remove (toFront);
			m_Data.Add (toFront);
		}

		if (dropDead != null)
		// Destroy an object if needed
		{
			m_Data.Remove (dropDead);
		}

		if (flipRepaint)
		// If some object just stopped being dragged, we should repaing for the state change
		{
			Repaint ();
		}
	}
}

MyMonoBehaviour.cs

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

public class MyMonoBehaviour : MonoBehaviour
{
	private List< DataObject > m_Data = new List< DataObject > ();
	private Rect dropTargetRect = new Rect (10.0f, 10.0f, 30.0f, 30.0f);

	void Awake ()
	{
		m_Data.Add (new DataObject ("One", 1, new Vector2 (20.0f * Random.Range (1.0f, 10.0f), 20.0f * Random.Range (1.0f, 10.0f))));
		m_Data.Add (new DataObject ("Two", 2, new Vector2 (20.0f * Random.Range (1.0f, 10.0f), 20.0f * Random.Range (1.0f, 10.0f))));
		m_Data.Add (new DataObject ("Three", 3, new Vector2 (20.0f * Random.Range (1.0f, 10.0f), 20.0f * Random.Range (1.0f, 10.0f))));
		m_Data.Add (new DataObject ("Four", 4, new Vector2 (20.0f * Random.Range (1.0f, 10.0f), 20.0f * Random.Range (1.0f, 10.0f))));
		m_Data.Add (new DataObject ("Five", 5, new Vector2 (20.0f * Random.Range (1.0f, 10.0f), 20.0f * Random.Range (1.0f, 10.0f))));
	}

	public void OnGUI ()
	{
		DataObject toFront, dropDead;
		Color color;

		GUI.Box(dropTargetRect, "Die");

		toFront = dropDead = null;
		foreach (DataObject data in m_Data)
		{
			color = GUI.color;

			if (data.Dragging)
			{
				GUI.color = dropTargetRect.Contains (Event.current.mousePosition) ? Color.red : color;
			}

			data.OnGUI ();

			GUI.color = color;

			if (data.Dragging)
			{
				if (m_Data.IndexOf (data) != m_Data.Count - 1)
				{
					toFront = data;
				}
			}
		}

		if (toFront != null)
		// Move an object to front if needed
		{
			m_Data.Remove (toFront);
			m_Data.Add (toFront);
		}
	}
}