I was really hoping ZFS would have taken off. It offered the same type of storage pooling that mhddfs offers, and also offered redundancy if you wanted. It's very similar to how larger enterprise storage devices work, like the EVA series I supported at HP.

Some RAID controllers do offer a similar option, usually termed JBOD (Just a Bunch Of Disks). They would operate at a block level though instead of file level, and just sit there filling up the first disk, then rolling over to the second, third, etc.