It is very hard to find a fit-all client need's intranet product. One of the best one in the market is the Intranet Dashboard - ID (www.intranetdashboard.com). This product has won many awards and you will be impress of the size of their clients.

ID might give you more than you need but you can simple turn-on/off the features. I do think it is a bit pricy, but it depends on your clients at the end.

-Juan

I've used the open source CMS e107 for a large (600+ users) intranet site. There are a lot of plugins that can be tweaked to do different functionality but you need to have/be a internet functional designer /coordinator. e107 is very easy to use and up to date on possible leaks. Much better then Joomla in my humble opinion. Good support too in various languages.