Add a custom bookmarks MultiItem to the ToolbarControl
SpatialBookmarks.cs
// Copyright 2012 ESRI
// 
// All rights reserved under the copyright laws of the United States
// and applicable international laws, treaties, and conventions.
// 
// You may freely redistribute and use this sample code, with or
// without modification, provided you include the original copyright
// notice and use restrictions.
// 
// See the use restrictions.
// 

using System;
using ESRI.ArcGIS.SystemUI;
using ESRI.ArcGIS.esriSystem;
using ESRI.ArcGIS.Controls;
using ESRI.ArcGIS.Carto;

namespace MultiItemBookmarks
{

  public class SpatialBookmarks : IMultiItem
  {
    private IHookHelper m_HookHelper;

    public SpatialBookmarks()
    {
      m_HookHelper = new HookHelperClass();
    }
  
    public int get_ItemBitmap(int index)
    {
      return 0;
    }
  
    public string get_ItemCaption(int index)
    {  
      //Get the bookmarks of the focus map
      IMapBookmarks mapBookmarks = (IMapBookmarks) m_HookHelper.FocusMap;

      //Get bookmarks enumerator
      IEnumSpatialBookmark enumSpatialBookmarks = mapBookmarks.Bookmarks;
      enumSpatialBookmarks.Reset();

      //Loop through the bookmarks to get bookmark names
      ISpatialBookmark spatialBookmark = enumSpatialBookmarks.Next();

      int bookmarkCount = 0;
      while (spatialBookmark != null)
      {
        //Get the correct bookmark
        if (bookmarkCount == index)
        {
          //Return the bookmark name
          return spatialBookmark.Name;
        }
        bookmarkCount = bookmarkCount + 1;
        spatialBookmark = enumSpatialBookmarks.Next();
      } 
      
      return "";
    }
  
    public bool get_ItemChecked(int index)
    {
      return false;
    }
  
    public bool get_ItemEnabled(int index)
    {
      return true;
    }
  
    public void OnItemClick(int index)
    {
      //Get the bookmarks of the focus map
      IMapBookmarks mapBookmarks = (IMapBookmarks) m_HookHelper.FocusMap;

      //Get bookmarks enumerator 
      IEnumSpatialBookmark enumSpatialBookmarks = mapBookmarks.Bookmarks;
      enumSpatialBookmarks.Reset();

      //Loop through the bookmarks to get bookmark to zoom to
      ISpatialBookmark spatialBookmark = enumSpatialBookmarks.Next();

      int bookmarkCount = 0;
      while (spatialBookmark != null)
      {
        //Get the correct bookmark
        if (bookmarkCount == index)
        {
          //Zoom to the bookmark
          spatialBookmark.ZoomTo(m_HookHelper.FocusMap);
          //Refresh the map
          m_HookHelper.ActiveView.Refresh();
        }
        bookmarkCount = bookmarkCount + 1;
        spatialBookmark = enumSpatialBookmarks.Next();
      } 

    }
  
    public int OnPopup(object hook)
    {
      m_HookHelper.Hook = hook;

      //Get the bookmarks of the focus map
      IMapBookmarks mapBookmarks = (IMapBookmarks) m_HookHelper.FocusMap;

      //Get bookmarks enumerator
      IEnumSpatialBookmark enumSpatialBookmarks = mapBookmarks.Bookmarks;
      enumSpatialBookmarks.Reset();

      //Loop through the bookmarks to count them
      ISpatialBookmark spatialBookmark = enumSpatialBookmarks.Next();

      int bookmarkCount = 0;

      while (spatialBookmark != null) 
      {
        bookmarkCount = bookmarkCount + 1;
        spatialBookmark = enumSpatialBookmarks.Next();
      } 

      //Return the number of multiitems
      return bookmarkCount;
    }
  
    public string Caption
    {
      get
      {
        return "Spatial Bookmarks";
      }
    }
  
    public int HelpContextID
    {
      get
      {
        return 0;
      }
    }
  
    public string HelpFile
    {
      get
      {
        return null;
      }
    }
  
    public string Message
    {
      get
      {
        return "Spatial bookmarks in the focus map";
      }
    }
  
    public string Name
    {
      get
      {
        return "Spatial Bookmarks";
      }
    }
  }
}