BuddyPress.org

Some stuff already available by using WPMU plugins:

– sitewide feed

– recent posts across the system

– latest blogs or members

– member list

all over at http://wpmudev.org/plugins.php

Remember, Buddypress is a plugin for MU – mind you, a really extensive plugin. Other sitewide features can be added the same way they get added in WPMU, it’s just a matter of figuring out if what the end result you want is pulling directly from a BP function or a regular MU function.