Just launched this week, Yammer. What the heck is Yammer? Well, think if it as a semi-private version of Twitter. Yammer has all the features of Twitter (mini-blog type updates, choosing what users to follow, integration with SMS and IM), but with one major difference. 'Tweets' are generally viewable by any Twitter user. Any one can follow you on Twitter. Making your Tweets private is possible, but most users don't bother.
Yammer creates mini-networks of users sharing the same e-mail domain. That is only users with authenticated @acme.com email addresses can follow and see the updates of other folks from the Acme Company. So it effectively becomes a mini-Twitter just for the people in your company.
Yammer has a couple of other features currently missing in Twitter, an easy way to view threaded discussions and the ability to 'follow' a hashtag or topic instead of a user. Yammer also offers companies the ability for more robust administration capabilities for a fee of $1 user/month.
Many people I have talked to never could grasp the value of Twitter in the enterprise, (too many 'I just woke up and I'm tired' posts) may see more meaning in Yammer, where at least you know that all the traffic is from your own organization.
A couple of quick ideas as to how an HR department could use Yammer:
What other uses of Yammer can you think of?