Useful script to display GUI resources available by default.

GUIResourcesWindow.cs

// Adapted from http://wiki.unity3d.com/index.php/Show_Built_In_Resources

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

namespace PainfulSmile
{
    public sealed class GUIResourcesWindow : EditorWindow
    {
        private enum ResourcesType
        {
            Styles,
            Icons
        }

        private enum SizeType
        {
            Original,
            Expanded
        }

        private bool _forceExpand;
        private int _iconIndex;
        private int _styleIndex;
        private string _search = "";
        private ResourcesType _resourcesType;
        private SizeType _sizeType;
        private Vector2 _iconsScrollPosition;
        private Vector2 _stylesScrollPosition;
        private Texture2D[] _icons;

        [MenuItem("Tools/GUI Resources")]
        private static void OpenMenuItem()
        {
            List<Texture2D> __listIcons = new List<Texture2D>(Resources.FindObjectsOfTypeAll(typeof(Texture2D)) as Texture2D[]);
            __listIcons.Sort((iconA, iconB) => string.Compare(iconA.name, iconB.name, System.StringComparison.OrdinalIgnoreCase));

            GUIResourcesWindow __window = GetWindow<GUIResourcesWindow>(false, "GUI Resources");
            __window._icons = __listIcons.ToArray();
            __window.minSize = new Vector2(512, 256);
            __window.Show();
        }

        private void OnGUI()
        {
            GUILayout.Space(4);

            using (new GUILayout.HorizontalScope())
            {
                EditorGUIUtility.labelWidth = 64;

                _resourcesType = (ResourcesType)EditorGUILayout.EnumPopup("Resources", _resourcesType);

                EditorGUIUtility.labelWidth = 48;

                _sizeType = (SizeType)EditorGUILayout.EnumPopup("Size", _sizeType);
            }

            GUILayout.Space(4);

            _search = EditorGUILayout.TextField(_search);

            GUILayout.Space(4);

            Rect __buttonRect = new Rect();
            __buttonRect.x = (position.width / 2) + 8;
            __buttonRect.width = (position.width / 2) - 12;
            __buttonRect.height = GUI.skin.button.CalcSize(new GUIContent("Copy")).y;
            __buttonRect.y = position.height - __buttonRect.height - 4;

            switch (_resourcesType)
            {
                case ResourcesType.Styles:
                    CheckKeyEvent(ref _styleIndex, ref _stylesScrollPosition, __buttonRect.height, GUI.skin.customStyles.Length);

                    using (new GUILayout.HorizontalScope())
                    {
                        using (new GUILayout.VerticalScope(GUILayout.Width(position.width / 2)))
                        {
                            using (var scrollScope = new GUILayout.ScrollViewScope(_stylesScrollPosition, GUI.skin.textField))
                            {
                                _stylesScrollPosition = scrollScope.scrollPosition;

                                for (int i = 0; i < GUI.skin.customStyles.Length; i++)
                                {
                                    if (_search != "" && !GUI.skin.customStyles[i].name.ToLower().Contains(_search.ToLower()))
                                    {
                                        continue;
                                    }

                                    if (GUILayout.Toggle(_styleIndex == i, GUI.skin.customStyles[i].name, EditorStyles.toolbarButton))
                                    {
                                        _styleIndex = i;
                                    }
                                }
                            }
                        }

                        Rect __styleRect = new Rect();
                        __styleRect.x = (position.width / 2) + 48;
                        __styleRect.y = 48;
                        __styleRect.width = (position.width / 2) - 52;

                        switch (_sizeType)
                        {
                            case SizeType.Original:
                                __styleRect.height = GUI.skin.customStyles[_styleIndex].CalcHeight(new GUIContent("Style Label"), __styleRect.width);
                                break;

                            case SizeType.Expanded:
                                __styleRect.height = position.height - __buttonRect.height - __styleRect.y - 8;
                                break;
                        }

                        EditorGUI.LabelField(__styleRect, "Style Label", GUI.skin.customStyles[_styleIndex]);

                        if (GUI.Button(__buttonRect, "Copy"))
                        {
                            CopyText("new GUIStyle(\"" + GUI.skin.customStyles[_styleIndex].name + "\")");
                        }
                    }
                    break;

                case ResourcesType.Icons:
                    CheckKeyEvent(ref _iconIndex, ref _iconsScrollPosition, __buttonRect.height, _icons.Length);

                    using (new GUILayout.HorizontalScope())
                    {
                        using (new GUILayout.VerticalScope(GUILayout.Width(position.width / 2)))
                        {
                            using (var scrollScope = new GUILayout.ScrollViewScope(_iconsScrollPosition, GUI.skin.textField))
                            {
                                _iconsScrollPosition = scrollScope.scrollPosition;

                                for (int i = 0; i < _icons.Length; i++)
                                {
                                    if (string.IsNullOrEmpty(_icons[i].name) || (_search != "" && !_icons[i].name.ToLower().Contains(_search.ToLower())))
                                    {
                                        continue;
                                    }

                                    if (GUILayout.Toggle(_iconIndex == i, new GUIContent(_icons[i].name), EditorStyles.toolbarButton))
                                    {
                                        _iconIndex = i;
                                    }
                                }
                            }
                        }

                        Rect __textureRect = new Rect();

                        switch (_sizeType)
                        {
                            case SizeType.Original:
                                __textureRect = GUILayoutUtility.GetRect(_icons[_iconIndex].width, _icons[_iconIndex].width, _icons[_iconIndex].height, _icons[_iconIndex].height, GUILayout.ExpandHeight(false), GUILayout.ExpandWidth(false));
                                __textureRect.x += (position.width / 4) - (__textureRect.width / 2);
                                __textureRect.y = 12 + (position.height / 2) - (__textureRect.height / 2);
                                break;

                            case SizeType.Expanded:
                                __textureRect.x = (position.width / 2) + 8;
                                __textureRect.y = 48;
                                __textureRect.width = (position.width / 2) - 12;
                                __textureRect.height = position.height - __buttonRect.height - __textureRect.y - 8;
                                break;
                        }

                        EditorGUI.DrawTextureTransparent(__textureRect, _icons[_iconIndex]);

                        if (GUI.Button(__buttonRect, "Copy"))
                        {
                            CopyText("EditorGUIUtility.FindTexture(\"" + _icons[_iconIndex].name + "\")");
                        }
                    }
                    break;
            }

            GUILayout.Space(4);
        }

        private static void CopyText(string text)
        {
            TextEditor __editor = new TextEditor();
            __editor.text = text;
            __editor.SelectAll();
            __editor.Copy();
        }

        private void CheckKeyEvent(ref int index, ref Vector2 scrollPosition, float itemHeight, int itemCount)
        {
            if (Event.current != null && Event.current.type == EventType.KeyDown)
            {
                switch (Event.current.keyCode)
                {
                    case KeyCode.UpArrow:
                        index = Mathf.Max(index - 1, 0);

                        if (index * itemHeight < scrollPosition.y)
                        {
                            scrollPosition.y -= itemHeight;
                        }

                        Repaint();
                        break;

                    case KeyCode.DownArrow:
                        index = Mathf.Min(index + 1, itemCount - 1);

                        if (index * itemHeight > scrollPosition.y + position.height - itemHeight - 48)
                        {
                            scrollPosition.y += itemHeight;
                        }

                        Repaint();
                        break;

                    case KeyCode.Home:
                        index = 0;
                        scrollPosition.y = 0;
                        Repaint();
                        break;

                    case KeyCode.End:
                        index = itemCount - 1;
                        scrollPosition.y = (itemHeight * itemCount) - ((position.height - 48) / itemHeight);
                        Repaint();
                        break;

                    case KeyCode.PageUp:
                        {
                            int __previousIndex = index;
                            index = Mathf.Max(index - Mathf.RoundToInt((position.height - 48) / itemHeight), 0);
                            scrollPosition.y += (index - __previousIndex) * itemHeight;
                        }

                        Repaint();
                        break;

                    case KeyCode.PageDown:
                        {
                            int __previousIndex = index;
                            index = Mathf.Min(index + Mathf.RoundToInt((position.height - 48) / itemHeight), itemCount - 1);
                            scrollPosition.y += (index - __previousIndex) * itemHeight;
                        }

                        Repaint();
                        break;

                    default:
                        break;
                }
            }
        }
    }
}