Step Two Designs, in conjunction with the IA Institute, have just released a free Intranet Review Toolkit
This toolkit provides intranet managers and designers with an easy-to-use method to assess the strengths and weaknesses of their intranets.
While I don’t necessarily agree with all of the guidelines (or applying them blindly without considering your specific circumstances), it’s a good starting point to get you thinking about the various aspects of your intranet.