<UserControl x:Class="MySystem.Common.Controls.DropDownPanel"
             …              
             x:Name="This">
    <Grid>
        <ToggleButton x:Name="ShowPopupButton"/>
        <Popup
            PlacementTarget="{Binding ElementName=ShowPopupButton}"
            Placement="{Binding PopupPlacement, ElementName=this}"
            PopupAnimation="Slide"
            AllowsTransparency="True"
            Focusable="True" 
            StaysOpen="False"
            IsOpen="{Binding IsChecked, ElementName=ShowPopupButton }">
            <Border Background="#FFE3EAF3" BorderThickness="1" Padding="2">
                <Grid>
                    <ItemsControl ItemsControl.ItemsSource="{Binding Path=PanelItems,ElementName=This }">
                        <ItemsControl.ItemsPanel>
                            <ItemsPanelTemplate>
                                <Grid>

                                </Grid>
                            </ItemsPanelTemplate>
                        </ItemsControl.ItemsPanel>
                    </ItemsControl>
                </Grid>
            </Border>
        </Popup>
    </Grid>
</UserControl>

  public static readonly DependencyProperty PanelItemsProperty = DependencyProperty.Register("PanelItems"
            , typeof(ObservableCollection<UIElement>)
            , typeof(DropDownPanel)
            , new PropertyMetadata(new ObservableCollection<UIElement>()));
        public ObservableCollection<UIElement> PanelItems
        {
            get
            {
                return (ObservableCollection<UIElement>)GetValue(PanelItemsProperty);
            }
            set
            {
                SetValue(PanelItemsProperty, value);
            }
        }

<Window x:Class="MySystem.UI.View.Window1"
              ….
             xmlns:controls ="clr-namespace:MySystem.Common.Controls;assembly=GoldAccountingSystem.Common.Controls">
    <Grid>
        <controls:DropDownPanel>
            <controls:DropDownPanel.PanelItems>
                    !--Put Controls Here--!
            </controls:DropDownPanel.PanelItems>
        </controls:DropDownPanel>
    </Grid>
</Window>

namespace MySystem.Common.Controls
{
    public class UIElementCollection : ObservableCollection<UIElement> { }
}

<Window x:Class="MySystem.UI.View.Window1"
              ….
             x:Name="This"
             xmlns:controls ="clr-namespace:MySystem.Common.Controls;assembly=GoldAccountingSystem.Common.Controls">
    <Grid>
        <commonControls:DropDownPanel>
            <commonControls:DropDownPanel.PanelItems>
                <commonControls:UIElementCollection>
                    !--Put Controls Here--!
                </commonControls:UIElementCollection>
            </commonControls:DropDownPanel.PanelItems>
            </commonControls:DropDownPanel>
    </Grid>
</Window>