The User Activity plugin provides you with the ability to collects user activity data as number of users in last:

  1. 5 minutes
  2. Hour
  3. Day
  4. Week
  5. Month

This plugin also allows you to display the number of users currently online anywhere on your main blog (or any other blog).

Simply drop blog-activity.php into mu-plugins.

Now you can run reports on your blogs and users via your new User Activity tab in Site Admin.

User Activity Page

Other plugin options for monitoring activity on your site include:

  1. Blog Activity plugin - Collects data on how many blogs updated in the past “minutes”, “hour”, “day”, and “month”.

  2. Reports plugin -  Displays post and comment activity per blog and per user

    3. Reports plugin

  3. Google Analytics for WordPress MU - Sitewide AND single blog solution! plugin - this neat plugin allows you to simply enter your 'UA' code into a new field in Site Admin > Options AS WELL AS allowing your users to enter their code in Settings > Google Analytics..

Neato, huh?