Yammer.com has been pitched as Twitter with security controls. That is a micro-blogging/updating service where to can limit the "followers" to ensure internal security. I it supposed to be better suited to corporate users. I have just started to trial it with my team. Will let you know how it goes. I first heard of Yammer while watching the TechCrunch50 list, looking for travel companies. Yammer was the TechCrunch50 winner.
Of course the day before I decide to kick off the trial the first piece of bad news for Yammer (capacity issues) hits the interwebosphere (TechCrunch story here).