Top link bar sub-site drop-down list in SharePoint Foundation 2010

You could use this method to custom your drop-down list top link bar in SharePoint 2010,Easy and Simple SharePoint 2010 Dropdown Navigation bar that works no scripts, no web.config, works on hosted providers.

Here is my code in a custom Master.page I used SharePoint Designer to set the DataSourceID=SiteMapDataSource1 – in the GUI click the TopMenuNav bar click the top-right > arrow and then “New Data Source” use the default name.

Code should look like this. I also changed the SPSiteMapProvider under
SiteMapProvider in both ID= under AspMenu and SiteMapProvider under
aspSiteMapDataSource.

<SharePoint:AspMenu
     ID="SPSiteMapProvider"
     Runat="server"
     EnableViewState="false"
     DataSourceID="SiteMapDataSource1"
     UseSimpleRendering="true"
     UseSeparateCss="false"
     Orientation="Horizontal"
     StaticDisplayLevels="2"
     MaximumDynamicDisplayLevels="4"
     SkipLinkText=""
     CssClass="s4-tn"/>
    <asp:SiteMapDataSource runat="server" ID="SiteMapDataSource1" />
    <SharePoint:DelegateControl runat="server" ControlId="TopNavigationDataSource" Id="topNavigationDelegate">
        <Template_Controls>
            <asp:SiteMapDataSource
             ShowStartingNode="True"
             SiteMapProvider="SPSiteMapProvider"
             id="topSiteMap"
             runat="server"
             StartingNodeUrl="sid:1002"/>
        </Template_Controls>
    </SharePoint:DelegateControl>

Then you will find your sharepoint site’s top link bar like this:

I think Microsoft they should add this feature by default.