http://code.trac.wordpress.org/
All sorts of goodies released under the GPL from the folks behind WordPress. Including jobs, stats stuff, mediawiki login integration, wpshell, and lots more.
http://db-tools.wpmudev.org/db_servers.php
Tools to automatically generate multidb related code such as create database queries, db-list.php files and so on.
http://getshopped.com/
GetShopped is a WordPressMU based ecommerce platform. It's initially free (limited to 200 transactions / month). No prices are available for the upgrades.
http://incsub.com/
Incsub are a WordPress MU development company. They manage EduBlogs.com amongst others.
http://moodle.org/
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.
http://wpmu.org/
WordPress MU community site by the people at Incsub (professional WPMU developers).
http://wpmu.org/scaling-wordpress-wpmu-buddypress-like-edublogs/
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.
http://wpmudev.org/
WPMU Dev is a resource for WordPressMU (MultiUser) plugins and other goodies. It also offers a commercial service providing support and other things.
http://wpmudev.org/project/wp-autoblog
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.
http://www.blojsom.com/
Blojsom is a java blog platform. Multi-blog, multi-user blogging platform. It powers Apple's Mac OS X Tiger Server Weblog Server.
http://www.slatetechpdx.com/
Web development company apparently working in the WPMU space.