QNAP devices. They make simple nas devices to insane. They are the swiss army knife of NAS devices. The list of options on those devices can make your head spin. And they support every RAID option even JBOD.
We went with Drobo because it is a simple device that met our connect requirements and their obnoxiously named "BeyondRAID" is probably the best RAID solution around.
I really want to check out QNAP, but at the devices that met our needs just had too much options. It would have been too easy for things to go wrong. Especially with our luddite techs, don't ask how that happens.
It's called a plain old NAS. With the Drobo, you're paying for the RAID tech and cheap NAS don't have it.
Take a look at the HP Home Server. It is a windows-based, stand-alone server thats designed to be a media/file server. You can add an additional internal drive plus USB drives as will. It can run, I believe as an iTunes server (someone correct me if I'm wrong :) Runs great in a a mac & windows network. In fact, it was voted "best of show" at the last Mac World shows. Pretty impressive for a windows based box.
This is one I would have gotten if I didn't get a Drobo. It's not as expensive as the drobe and and a little more flexible in what you could do with it (iTunes, etc - and don't get me started on the Drobo + Droboshare bunk - I have a droboshare and it not really usable - but we'll save that for another thread).
Did you consider a Synology Diskstation? Ok, it might not be cheap, but it is a very solid, well build and multifunctional solution, which I'm using for quite some time now.
// Remo Knops
The only other way to do the 'flexible expansion' that you want is with xfs. Drobo is the only commercial product that allows you to add drives to an array without backing everything up and having to completely re-build the set.