CSS style dropdown/flyout menus for sub-categories?

I know there is a checkbox in the WP category widget to make it a dropdown, but that's not what I'm after.

I would like the categories in my sidebar that do have sub-categories to have a dropdown that expands on rollover of the parent category, like a regular dropdown menu. I am planning a category that has a dozen or so sub categories, and the only way to effectively display those will be a dropdown/flyout menu. I guess since it is a vertical navigation area it would be more of a flyout than a dropdown.

So, is this possible to do? I figure if worst comes to worst I can hard code one with html/css using the text widget, but I'd love to stick with a dynamic one, so I don't have to manually change it when the categories change.