ItemsControl with horizontal orientation

While the promoted answer is great, here's an alternative if you want the items to stretch.

<ItemsControl.ItemsPanel>                              
    <ItemsPanelTemplate>
        <UniformGrid Rows="1" />
    </ItemsPanelTemplate>
</ItemsControl.ItemsPanel>   

Simply change the panel used to host the items:

<ItemsControl ...>
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <StackPanel Orientation="Horizontal"/>
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
</ItemsControl>