This is a category archive plugin. It displays a list of all categories, along with the first x posts for each category.
The plugin lives at http://www.neato.co.nz/plugins/category_archive.txt.
To install, copy the contents of that file to your plugins directory; enable the plugin; then make a call to the get_category_archive($limit, $parent) function.
$limit specifies the number of posts to get for each category; and $parent specifies the parent id. If the $parent parameter is left blank, then all of the categories will be retrieved. Alternately, if a category id is specified, only the categories under that category will be retrieved.
The output takes the form of a set of divs, with the specified CSS class names.