remm

This plugin provide widget that display a list with categories and posts in your sidebar. 
Widget can save the opened categories, and use cache.

Wp Ajax Tree

This plugin provide a widget that display a category and pages tree on your sidebar, using ajax to collapse categories.

Brian

ThemeBoy

Displays an interactive visual sitemap in admin. A nice touch for sites delivered to clients, and useful for managing a site as it gives a visual overview of all pages, tags, and categories.

Visual Sitemap

Display an interactive visual sitemap of pages, tags, and categories in admin.

Wp Ajax Tree vs Visual Sitemap