The Multisite administration plugin adds addition columns to the sites, plugins and themes tables in the network admin interface.
On the sites table, two additional columns are added to allow the admins to easily view the theme of the site, and also any plugins that are enabled.
On the themes table, there is an additional column added which easily allows the adminstrator to see all sites that are actively using that theme.
On the plugins table, there is an additional column added which easily allows the administartor to see all sites that are actively using that plugin.