Yammer is being developed as Twitter for companies.
According to TechCrunch:
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. The basic Yammer service is free. Companies can pay to claim and administer their networks.
They also have a blog - see http://blog.yammer.com/blog/