Yammer - "Twitter" For Businesses

Check it out:

Yammer.com

They say: " Yammer is a tool for making companies and organizations more productive through the exchange of short frequent answers to one simple question: 'What are you working on?'

As employees answer that question, a feed is created in one central location enabling co-workers to discuss ideas, post news, ask questions, and share links and other information. Yammer also serves as a company directory in which every employee has a profile and as a knowledge base where past conversations can be easily accessed and referenced.

Anyone in a company can start their Yammer network and begin inviting colleagues. The privacy of each network is ensured by limiting access to those with a valid company email address. Information is never shared with third parties..."