Just like in the "Multiple Choice" fields and "Checkbox fields there this option.
This would be incredibly useful in a drop-down field as well. Let me explain. Right now, to sorta work around it, I've created an option within the dropdown called "Other / Not Listed" and if a user selects that then I set some logic to show a text box where the user can type in an option not listed in the drop-down.
Problem is when running reports, or looking at the manage entires page, you have to display another field to see what the user wrote in the "Other / Not Listed" field. When you have 5-10 dropdowns in a form that need a "Other / Not Listed" option your having to display 15-20 fields to get the info you need in a report or entries page! With "Multiple Choice" or "Checkbox" fields it keeps that data and stores it within the same table.
Well, you say "Why not make it a multiple choice field and be done?" some of our drop-downs can have 100's of options and would fill a screen up 10 times over...which was why the drop-down field was made. I don't see it being real difficult to add this to a upcoming version of Machform hope that it would considered.