Simple (I think) Horizontal Line in WPF?

How about add this to your xaml:

<Separator/>

I had the same issue and eventually chose to use a Rectangle element:

<Rectangle HorizontalAlignment="Stretch" Fill="Blue" Height="4"/>

In my opinion it's somewhat easier to modify/shape than a separator. Of course the Separator is a very easy and neat solution for simple separations :)

Use a Border of height 1 and don't set the Width (i.e. Width = Auto, HorizontalAlignment = Stretch, the default)