Merlin Validator gives window forms validation similar to the web form validation controls. You can group validators by button, and use regex or range validation. The validator control is a component control, so you only need to add it to the form once. It works similar to a tooltip, in that it extends the existing controls on the form. The screenshots below show the validator in the IDE, and then running (after the user has clicked the submit button with any empty field).
||Free to try
||Windows 2000/XP/2003 Server/Vista