Developed for WordPress MU, this plugin gives you the ability to set custom ‘description’ and ‘keywords’ meta tags in the ‘head’ area of your site. The plugin is helpful because it gives the site admin the ability to set keywords and description for every page on every blog (site wide) and then it allows each individual blog admin to set their own description and keywords. The plugin then combines them into one before inserting the result into the head of your website.
To adjust the site wide keywords and description, sign in as a site administrator and go to the settings tab in the admin area. To adjust the keywords and description for a single blog, log in as a single blog admin (without site admin privileges) and navigate to the settings tab in the backend.
Download it here: WP Plugin Repository