[Unity3D Editor] PropertyDrawer for [EnumFlag] attribute - draws an expandable vertical column of toggle buttons filled by your Enum names.

EnumFlagDrawer.cs

using UnityEditor;
using UnityEngine;

[CustomPropertyDrawer(typeof(EnumFlagAttribute))]
public class EnumFlagDrawer : PropertyDrawer
{
	private static bool foldout = false;

	public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
	{
		position.height = 16f;

		foldout = EditorGUI.Foldout( position, foldout, label );

		if (foldout)
		{
			int buttonsIntValue = 0;
			int enumLength = property.enumNames.Length;
			bool[] buttonPressed = new bool[enumLength];
			float buttonsLeft = position.x + EditorGUIUtility.labelWidth;
			float buttonWidth = position.width - EditorGUIUtility.labelWidth;
			float buttonHeight = EditorGUIUtility.singleLineHeight;

			EditorGUI.LabelField( new Rect( position.x, position.y, EditorGUIUtility.labelWidth, position.height ), label );

			EditorGUI.BeginChangeCheck();

			position.height = buttonHeight * enumLength;

			for (int i = 0; i < enumLength; i++)
			{
				// Check if the button is/was pressed 
				if ((property.intValue & (1 << i)) == 1 << i)
				{
					buttonPressed[i] = true;
				}

				Rect buttonPos = new Rect( buttonsLeft, 16f + position.y + i * buttonHeight, buttonWidth, buttonHeight );

				buttonPressed[i] = GUI.Toggle( buttonPos, buttonPressed[i], property.enumNames[i], "Button" );

				if (buttonPressed[i])
					buttonsIntValue += 1 << i;
			}

			if (EditorGUI.EndChangeCheck())
			{
				property.intValue = buttonsIntValue;
			}
		}
	}

	public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
	{
		return base.GetPropertyHeight( property, label ) +
				(foldout
					? EditorGUIUtility.singleLineHeight * property.enumNames.Length
					: 0f);
	}
}