I assumed that you could not use Google Analytics for your intranet. Apparently I was wrong.
Sadalit Van Buren takes you through the steps on her A Matter of Degree blog: How to Use Google Analytics with Sharepoint (MOSS 2007).
Google Analytics is not the most robust tools for measuring traffic. But these are tight economic times and it is good to have a free tool.
I use Google Analytics to view trends on this blog and some other sites I control. It does a great job of showing trends and the most viewed pages.