Announced only a couple days ago, has plans to release Chatter.  Chatter is a new enterprise social engine.  I like to think of it as a combination of IM and RSS feeds into a single platform that's security controlled and handled inside the Cloud.  So, it's more secure and allows the organization to communicate freely internally without risking exposure of confidential information.

Developers are going to like the new Chatter API.  On the surface it seems to be fairly straightforward with only 5 different objects in the data model (User, FeedPost, NewsFeed/Object, FeedTrackedChanges, & FeedComments).  Chatter will support adding content to the messages sent as well as sending messages from Profile/User, objects and feeds.

The next release Spring '10 is slated to include User Profiles, Post, Comments, & Feeds, Subscriptions, and Chatter only licenses ($50/month/user).  Future releases should include Questions & Answers and the new Groups.  (Safe Harbor applies.)

With Chatter, each user can have their own profile with information, a picture, followers, followees, status updates, etc.  Objects themselves can post to the feeds.  For instance when a deal is won, this information can be shared with everyone following the opportunity object record.  Security applies so if you don't see the opportunity, you don't see the feed.  Also, external systems can post feeds to be followed as well.

There's still a lot of questions around the API and we'll probably not know a whole lot more until it's officially released.