Basic service is free and lets you create up to 2 forums and 2 sub forums. You also get 100MB for file attachments (like pictures uploaded by users). You can also pay $25 to $75 a month for more storage space and the ability to create and manage more forums.
Yammer is opening up its microbogging platform. In "Yammer Community" people may now create a community without the requirement that an email address be associated with a particular domain.