WindStyle MultiSite Nav Bar plugin by xiaoshatian can show a navigation bar on every blog of your WordPress site.
All of blogs will show in this navigation bar. You can customize site logo and every blog can customize theme of their navigation bar.
This plugin uses the WordPress 3.0+ Multi-Site feature.
Download and info: http://wordpress.org/extend/plugins/windstyle-multisite-nav-bar-for-wordpress/