Yammer. It's a lot like Twitter, but for companies. So think of it as your own private Twitter network that only people with a pre-approved email address can view. Apparently it's all the rage over in the States, with a large number of major corporations already using it to waste time improve productivity levels. Besides the closed nature, one of the other distinguishing features is that you can tag your updates to indicate work on a particular project. So let's say I was working on a new project for one of our existing clients, Peugeot. Any update I made regarding the project I would tag 'Peugeot'. Assuming all colleagues working on the project did the same, we could all subscribe to the Peugeot feed by clicking the tag then clicking on the 'follow' button. That way, we can keep updates on our project separate from the general office banter.
As well as through the normal website, you can update your feed via an iPhone, Blackberry or desktop app or using IM, SMS or email.