When I was writing my Magic Marker application I wanted to make my notes input box spell checked. Turns out that this is really, really easy. This is the XAML for the notes texbox.
<TextBox Name="NotesTextBox" Width="500" Height="315" AcceptsReturn="True" ScrollViewer.CanContentScroll="True" TextWrapping="Wrap" SpellCheck.IsEnabled="True"></TextBox>
I just had to add the one extra property (that I've highlighted above) and it works a treat.