Google Analytics is a popular and mature tool for monitoring network activities on your site. Why not use it in Sharepoint? Dave Coleman and Mike Knowles provide good guides for doing that. There is a promising plugin to SP SPGoogleAnalytics which integrates both and provides an easy interface to put GA to your SharePoint installation and see the data directly on your intranet. It comes from codeplex: What do you think about spgoogleanalytics.