With next plugins, you can show invisible characters in Mac OS X with NSTextViews. Of course this also works for textarea controls in Cocoa targets.
The plugin can replace space, tab and new line characters with new characters and draw them. Also you can change the text color if needed, like use a dark gray or a red like in the example.
For this we extended NSLayoutManagerMBS class with new properties and also added methods to NSTextViewMBS to change the layout manager.