This widget displays top level categories in multiple columns.
After installation, just add the widget to desired sidebar or footer and optionally enter a custom title and the number of columns to be displayed.
By default two columns are used, but you can set any number of columns you like in the backend.
Also see details at http://arnowelzel.de/wp/en/projects/wordpress/multicolumn-category-widget.
This plugin is available for download for your site.