A few weeks ago, Frumph launched SocialPress as an informal project offering lightweight alternative plugins to BuddyPress. Alternatives are always nice.
My initial offering for SocialPress is a plugin which blends signup options by allowing only user signups, but logged in users can create blogs. The plugin is definitely lightweight and is designed to alter the behavior of the built-in signup: Require User Registration First (628). Just extract the zip and drop the plugin in the mu-plugins folder and set the site registration option to ‘Enabled. Blogs and user accounts can be created.’