Where to begin... and also depends on what you're talking about.
We have Sharepoint (ick) for sharing various things, which also links to our Change Control Processes, Billing Tools, Time Tracking Tools, etc, etc. Beyond that we have a few other areas that could be considered intranet including our Helpdesk area which tracks all kinds of information, a workgroup within our software product that tracks everything from client contacts to annoucements.
I'm actually in charge of bringing a lot of this stuff into a more centralized location (not Sharepoint). It's not an easy job.