FireStats is a comprehensive statistics package with support for various systems (WordPress, WordPress MU, trac, Drupal, Jumla and more). FireStats installation into WPMU is really simple, and it creates a single set of tables for the entire WPMU site, which are shared by all the blogs. Each user get their own statistics view in the dashboard, and the site administrator gets a complete administrative view, that enable him/her to administer FireStats. Some of the major features of FireStats are:

• Basic hit statistics (hits ever, in last 24 days etc)
• Recent popular pages
• Recent referrers
• Browser and OS trees, grouped by what matters
• IP to country - know where your visitors are surfing from
• Search engine keywords
• A detailed list of recent hits
• Excluding unwanted hits from the database
  • By IP address
  • By user-agent (to be used to filter bots)
  • By URL/Referrer (to filter referrer spam)
• Ajax UI, no need to reload the whole page - just press the ‘refresh statistics’ button to get the latest data.
• Designed to work on all browsers. (Tested on Firefox, Explorer, Opera)
• Compacts old data to improve performance and reduce used storage
• Bundled with translations to many languages (16 in the last count).

More information available at the FireStats site. Be sure to check the FireStats wiki page for WPMU.