A customized TreeView navigation control. For example, the NodeWrap has been set to True (it is False by default):

<div class="ms-quicklaunchouter" runat="server">
    <div class="ms-treeviewouter">
        <SharePoint:SPRememberScroll runat="server"
            id="RememberScroll1" 
            onscroll="javascript:_spRecordScrollPositions(this);"
            style="width: 220px;">
            <Sharepoint:SPTreeView runat="server"
                id="TreeView1" 
                ShowLines="false"
                DataSourceId="TreeViewDataSource1"
                ExpandDepth="0"
                SelectedNodeStyle-CssClass="ms-tvselected"
                NodeStyle-CssClass="ms-navitem"
                NodeStyle-HorizontalPadding="2"
                SkipLinkText=""
                NodeIndent="8"
                ExpandImageUrl="/_layouts/images/tvclosed.png"
                ExpandImageUrlRtl="/_layouts/images/tvclosedrtl.png"
                CollapseImageUrl="/_layouts/images/tvopen.png"
                CollapseImageUrlRtl="/_layouts/images/tvopenrtl.png"
                NoExpandImageUrl="/_layouts/images/tvblank.gif"
                NodeWrap="True">
            </Sharepoint:SPTreeView>
        </Sharepoint:SPRememberScroll>
    </div>
</div>
 
<SharePoint:SPHierarchyDataSourceControl runat="server" 
    id="TreeViewDataSource1" 
    RootContextObject="Web"
    ShowFolderChildren="true" 
    IncludeDiscussionFolders="false"/>

A minimal SPTreeView control:

<SharePoint:SPTreeView ID="TreeView1" DataSourceID="TreeViewDataSource1" runat="server" />
 
<SharePoint:SPHierarchyDataSourceControl runat="server" 
    id="TreeViewDataSource1" 
    RootContextObject="Web"
    ShowFolderChildren="true" 
    IncludeDiscussionFolders="false"/>

TreeNodeDataBound event:

// Reference namespaces.
using System.Web.UI;
using System.Web.UI.WebControls;
using Microsoft.SharePoint;
using Microsoft.SharePoint.Navigation;
using Microsoft.SharePoint.WebControls;
 
public partial class TestPage : LayoutsPageBase
{
    protected void Page_Load(object sender, EventArgs e)
    {
        // Register an event handler.
        TreeView1.TreeNodeDataBound += new TreeNodeEventHandler(TreeView1_TreeNodeDataBound);
 
        // Bind to a data source in order to invoke the TreeView1_TreeNodeDataBound event handler.
        TreeView1.DataBind();
    }
 
    // Implement the TreeView1_TreeNodeDataBound event handler.
    protected void TreeView1_TreeNodeDataBound(object sender, TreeNodeEventArgs e)
    {
        IHierarchyData d1 = (IHierarchyData)e.Node.DataItem;
        // ... use IHierarchyData properties: HasChildren, Item, Path, Type.
 
        ISPHierarchyData d2 = (ISPHierarchyData)e.Node.DataItem;
        // ... use ISPHierarchyData properties: EncodedName, ImageUrl, IsContextListNode, 
        // Name, NavigateUrl, ServerRelativeUrl.
    }
}

IHierarchyData and ISPHierarchyData interfaces:

public interface IHierarchyData
{
    // Methods
    IHierarchicalEnumerable GetChildren();
    IHierarchyData GetParent();
 
    // Properties
    bool HasChildren { get; }
    object Item { get; }
    string Path { get; }
    string Type { get; }
}
 
public interface ISPHierarchyData
{
    // Properties
    string EncodedName { get; }
    string ImageUrl { get; }
    bool IsContextListNode { get; }
    string Name { get; }
    string NavigateUrl { get; }
    string ServerRelativeUrl { get; }
}

Populating SPTreeView (TreeView1) recursively with sites and subsites:

protected void Page_Load(object sender, EventArgs e)
{
    BuildTree(SPContext.Current.Site);
}
 
private void BuildTree(SPSite site)
{
    TreeView1.Nodes.Clear();
 
    foreach (SPWeb web in site.RootWeb.Webs)
    {
        AddSitesAndLists(web, null);
        web.Dispose();
    }
 
    foreach (SPList list in site.RootWeb.Lists)
    {
        if (!list.Hidden)
        {
            TreeNode node = new TreeNode(list.Title, null, null, list.DefaultViewUrl, "_self");
 
            foreach (SPListItem folder in list.Folders)
            {
                TreeNode folderNode = 
                    new TreeNode(folder.Name, null, null, site.RootWeb.Url + "/" + folder.Url, "_self");
                node.ChildNodes.Add(folderNode);
            }
 
            TreeView1.Nodes.Add(node);
        }
    }
}
 
private void AddSitesAndLists(SPWeb web, TreeNode parentNode)
{
    TreeNode node = new TreeNode(web.Title, null, null, web.Url, "_self");
 
    if (parentNode == null)
        TreeView1.Nodes.Add(node);
    else
        parentNode.ChildNodes.Add(node);
 
    parentNode = node;
 
    foreach (SPWeb subsite in web.Webs)
    {
        AddSitesAndLists(subsite, parentNode);
        subsite.Dispose();
    }
 
    foreach (SPList list in web.Lists)
    {
        if (!list.Hidden)
        {
            node = new TreeNode(list.Title, null, null, list.DefaultViewUrl, "_self");
            parentNode.ChildNodes.Add(node);
 
            foreach (SPListItem item in list.Folders)
            {
                if (item.Folder != null)
                {
                    TreeNode folderNode = 
                        new TreeNode(item.Folder.Name, null, null, web.Url + "/" + item.Folder.Url, "_self");
                    node.ChildNodes.Add(folderNode);
                }
            }
        }
    }
}

A result of the above recursive procedure may look like that: