You can transform a default, single weblog WordPress installation into a multi-blog site with the help of PostGroups.
1.2.0 - July 4 - US Independence Day Release
[groupname]inside a post; it will get replaced with the name of the group to whom the post belongs, or with the blog title if the post is not in a group. Other shortcodes may be added in the future.
1.1.0 - July 1 - Canada Day Release