One example of creating resizable list boxes:
<Window x:Class="DragNdrop2resize.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="1*"/>
<ColumnDefinition Width="2*"/>
</Grid.ColumnDefinitions>
<ListBox Grid.Column="0">
<ListBoxItem Content="List box 1"/>
</ListBox>
<ListBox Grid.Column="1">
<ListBoxItem Content="List box 2"/>
</ListBox>
<GridSplitter Width="5"
HorizontalAlignment="Right"
VerticalAlignment="Stretch"
Background="Red" />
</Grid>
</Window>
As you can see I created column definitions, and added ListBoxes which belong to these columns (Grid.Column="1" for example), and on the end I added GridSpliter. Taken from here.
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2014-07-26 Update To have vertical resizable list boxes use something like:
<Window x:Class="WpfApplication13.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="60"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<ListBox Grid.Row="0">
<ListBoxItem Content="List box 1"/>
</ListBox>
<ListBox Grid.Row="1">
<ListBoxItem Content="List box 2"/>
</ListBox>
<GridSplitter Height="5"
HorizontalAlignment="Stretch"
VerticalAlignment="Bottom"
Background="Red" />
</Grid>
</Window>