Web development company apparently working in the WPMU space.
GetShopped is a WordPressMU based ecommerce platform. It's initially free (limited to 200 transactions / month). No prices are available for the upgrades.
WordPress MU community site by the people at Incsub (professional WPMU developers).
Incsub are a WordPress MU development company. They manage EduBlogs.com amongst others.
BuddyPress is a series of plugins which transform a vanilla WordPress MU installation into a social network. It's due for final release around Dec 2008.
WPMU Dev is a resource for WordPressMU (MultiUser) plugins and other goodies. It also offers a commercial service providing support and other things.