Windows Home Server (based on Windows Server 2003) which has been released in AU/NZ with the US and EU soon to follow. I've been a beta tester since b1 and it has been rock solid. When you run out of space you just put in a new drive and add it to the pool. For redundancy you have folder duplication which is great, because all files does not need online redundancy (like RAID5).

I think ScottC also runs WHS at home, so I am sure that he will give you his thoughts on it.