Simple CSS for widgets
by Gagan Goraya on WordPress.org
This plugin lets you specify CSS class(s) for widgets to apply your own CSS definitions to specific widgets.

Adds a custom "CSS class" text field where you can specify any CSS class for a particular widget.
This is a very slim piece of plugin that gives you incredible flexibility to apply your own CSS class(s) to any specific widget.
If this plugin is activated, it will present an additional ‘CSS Class’ field in every widget.
You can input any css class name or even more than one class name by separating each with a space.
If, somehow your widget is not wrapped inside an HTML block element (depends upon the theme you are
using), this plugin will intelligently wrap the widget with a DIV tag and apply to class to it.