For any SharePoint document library there is a default view called "Explorer View". This view displays the the contents of the document library as Windows explorer window that can be used to drag/drop or copy/paste files. This is very handy when transferring large sets of files/folders to a SharePoint document library.

Recently I implemented this view in a custom aspx page. Here's how I did it:

1. Add the explorer view container to your page. The explorer view uses an iframe as the container for the windows explorer window. Here is what the markup should look like.

   <iframe id="frmFolder" 
       name="frmFolder" 
       width="600" height="400" 
       src="/_layouts/blank.htm" class="ms-httpFolder">
   </iframe>

2. Initialized the explorer view container. Add the following code to your OnLoad page handler to initialize the iframe. In this example I am loading the root folder of the site collection. You can load any SharePoint folder you like.

   string js = string.Format(@"
           function navtoframe() {{ NavigateHttpFolderIfSupported('{0}{1}', 'frmFolder'); }} 
           _spBodyOnLoadFunctionNames.push('navtoframe');"
       , SPContext.Current.Site.Url
       , SPContext.Current.Site.RootWeb.RootFolder.ServerRelativeUrl);
   
   Page.ClientScript.RegisterStartupScript(GetType(), "frmFolder", js, true);

The NavigateHttpFolderIfSupported method is defined in the core.js file, which is a part of the SharePoint infrastructure and is associated with all SharePoint pages.