XAML ListBox bound to the output of the W3C Markup Validator. Originally http://lists.w3.org/Archives/Public/www-archive/2005Aug/0000.html (which also has a screenshot of the code in XamlPad).

validator-xaml.xaml

<Border xmlns="http://schemas.microsoft.com/winfx/avalon/2005" xmlns:x="http://schemas.microsoft.com/winfx/xaml/2005">
  <Grid>
    <Grid.Resources>
      <XmlDataSource x:Key="ValidatorData" Source="http://validator.w3.org/check?uri=http://www.google.com&amp;output=xml" XPath="result/messages/msg[position() &lt; 20]"/>
      <DataTemplate x:Key="ValidatorDataTemplate">
        <Grid>
          <ColumnDefinition Width="Auto" SharedSizeGroup="MessageLine" />
          <ColumnDefinition Width="Auto" SharedSizeGroup="MessageColumn" />
          <ColumnDefinition Width="*" SharedSizeGroup="MessageText" />
          <TextBlock Grid.Column="0" Margin="5,0,5,0" TextContent="{Binding XPath=@line}" />
          <TextBlock Grid.Column="1" Margin="5,0,5,0" TextContent="{Binding XPath=@col}" />
          <TextBlock Grid.Column="3" Margin="5,0,5,0" TextContent="{Binding XPath=.}" />
        </Grid>
      </DataTemplate>
    </Grid.Resources>
    <ListBox ItemsSource="{Binding Source={StaticResource ValidatorData}}" ItemTemplate="{StaticResource ValidatorDataTemplate}">
      <ListBox.Template>
        <ControlTemplate>
          <Grid Grid.IsSharedSizeScope="true">
            <RowDefinition Height="Auto" />
            <DockPanel>
              <Grid DockPanel.Dock="top">
                <ColumnDefinition Width="Auto" SharedSizeGroup="MessageLine" />
                <ColumnDefinition Width="Auto" SharedSizeGroup="MessageColumn" />
                <ColumnDefinition Width="*" SharedSizeGroup="MessageText" />
              </Grid>
              <StackPanel IsItemsHost="true" />
            </DockPanel>
          </Grid>
        </ControlTemplate>
      </ListBox.Template>
    </ListBox>
  </Grid>
</Border>