Provide option allowing to choose whether URLs should be spell checked.
When the spell-check find an error in a TextBox, the wrong word should be underlined with a red curly line.
Make the TextBox spell checker generic, so that controls that inherit from TextBox can be spell checked out of the box.
When the custom control for spell checking contains several TextBoxes, allow spell checking of them in turns, instead of opening a different spell checking dialog for each.