Which Multisite Plugins do you Recommend?

I have today installed a brand new MS network on a subdomain of my site. Everything is running fine and I have already been able to create three sites already. My main question is thus: what plugins do you recommend as being most useful on a MS network?

There are three that I am mostly looking for: 1) The ability to cross-post to related blogs accross the network, esp. the site’s main blog; 2) site wide tags; and 3) related posts across the network!

Any help you could give with this would be most appreciated. I hope to start writing my first posts on Sunday.

I wish you well.
Tor…