WPMU.org, possibly the biggest influence on the WordPress MU community, is celebrating the fact that they’ve created just about 100 plugins, themes, and videos specifically for WPMU.
I’ve been an on-and-off-again user of their products and services and think they do a great job. I’ll most likely be using their resources a bit more in the near future.
So, if you’re looking for WPMU support/plugins/themes then you’ve found it.