Interesting write up by James at Incsub on how they handle scale on edublogs. Some good advice although a little light on specifics, a good starting point for research.
All sorts of goodies released under the GPL from the folks behind WordPress. Including jobs, stats stuff, mediawiki login integration, wpshell, and lots more.
Tools to automatically generate multidb related code such as create database queries, db-list.php files and so on.
Web development company apparently working in the WPMU space.
Moodle is a course management system (CMS) - a free, Open Source software package designed using sound pedagogical principles, to help educators create effective online learning communities. Often compared against WPMU for education related stuff.
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.
Blojsom is a java blog platform. Multi-blog, multi-user blogging platform. It powers Apple's Mac OS X Tiger Server Weblog Server.
WP-AutoBlog is a plugin for WordPress that inserts posts based on RSS feeds. It has been used to generate a single WP site containing all the posts of a WPMU install to provide cross-site search / tagging / categories, etc. It could also power a planet.
WPMU Dev is a resource for WordPressMU (MultiUser) plugins and other goodies. It also offers a commercial service providing support and other things.