One of the first projects that I was tasked with at my current position was to create an Intranet for our company, I was all excited about the thought of my first project.  Through this project I found several plugins to be essential, they are:

Member List -  This allows you to create a list of the users in your wp database; however, this does require a lot of customization to get this to look how you want it to.

User Photo – this is a plugin that allows you to upload a photo from the user portfolio.  This also integrates into Member list easily.

NextGen Gallery - this allows you to create photo albums on your webpage

Event calendar/scheduler – this is a great plugin that allows you to update a calendar from the front end without going into the back end of wordpress.  This works for an intranet because you can allow all of your users or some of them add events.

Mingle Forum -  this is a forum program that allows members to have group conversations about various topics, and you are allowed to swap out different skins.