There is a dedicated server for alien swarm: http://developer.valvesoftware.com/wiki/Swarm_Dedicated_Server
However, there are no addon mods available to manage it and I don’t think you can even choose a server name either (for advertising purposes), So I dunno. Its only windows version as well.
I wonder if they will eventually charge for it. I have reached the highest prestige and there are still weapons that have not been unlocked. I don’t know what the future will be.