Hi all, I wanted to throw up the site I've been working on for a while which uses WPMU as it's core. It's *heavily* modified, but there are *no* core hacks (which I've quite proud of).
WPMU provides all internal user and "book" (blog) management, with extended information on profiles, the addition of a custom avatar system, very heavily modified posting process, plus a *bunch* of other stuff. Normal users can't even get at the traditional /wp-admin/ stuff, because it's totally replaced with our own dashboard system that handles only the bits that they need access to.
We've also got some forums which are powered by bbPress, and that's completely integrated includin avatars, profiles, etc.
Comments are welcome!