Where can I find an MU common navigation plugin

I've got a few mature Wordpress MU system running. I've looked high and low for a common navigation plugin and cannot find one.

On two of our MU systems we have so much information that we have broken down content that would normally work in a category into its own blog. For example, Coupons, News, Restaurants, Entertainment, Accounting, Medical etc. It is quite cumbersome to keep the navigation organized. I'm in the middle of a redesign right now so this would be a great time to find a plugin that could make the sub directory sites appear more uniform via Navigation.