Multiplayer is in beta right now, and should be coming out soon - if you want to try it early the beta is open - just send ScottLu a pm and ask to be added.
The addon missions are for single player as well as multiplayer - I highly recommend trying them if you want a new single player experience.
SHAMELESS PLUG: Why not give Icarus Burning a try? It's my campaign featuring six new missions from a Free Radical perspective. It features an unlockable "extreme" mode for extra playability, a scoring system, and some maps even have interactive terrain (like the ability to hide and ambush). It's pretty close to the top of the list - give it a shot!
OR, if you're not buying into my utterly shameless and egotistical self-hype, you could try missions by SoLan, Knelson, A_Shadow, Verdagon or Crackerz. Replicator Hazard by SoLan is a stunning three mission campaign you'll keep going back to!
Or you could pm about the beta. Sure there's always that...
