[shakna]

This still works today for me:

    Add-Type -AssemblyName PresentationFramework
    [XML]$form = @"
       <Window 
            xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
            Title="MainWindow" Height="450" Width="800">
        <Grid>
            
        </Grid>
        </Window>
    "@

    $NR = (New-Object System.Xml.XmlNodeReader $form)
    $window = [Windows.Markup.XamlReader]::Load($NR)

    $window.ShowDialog()