Co-op, a no-frills application with a simple user interface similar to Twitter's, is free for everyone.
CubeTree, recently acquired by SuccessFactors, is an on-demand enterprise collaboration suite available in free and premium versions. The free version includes user profiles, microblogging, file sharing, wikis, and 10MB per user.
Features: Suite includes standard microblogging characteristics. Status updates of 140 characters; a commenting feature (similar to commenting on activity streams on Facebook); feed filtering, which lets you choose who to follow and what feed items you'd like to receive; direct addressing (similar to Twitter's @name function); and integration with several third-party systems.
Features: Wikis, blogs, file repositories, event calendars, discussion boards, image galleries, collaboration spaces, status logs, people directory, crowd rating and voting and more.