Display All Non-Used Plugins

I think you can use this plugin:

http://wordpress.org/extend/plugins/wpmu-plugin-stats/screenshots/

Provides info to site admins as to which plugins are activated
sitewide, and which blogs plugins are activated on.

This plugin is using get_plugins()

http://codex.wordpress.org/Function_Reference/get_plugins

and fetches get_option('active_plugins') for each blog to compare.

It uses switch_to_blog() on each blog so it might be slower on large multisites, but I think this function got faster after the wordpress 3.5 update, according to