StableBit DrivePool - State of the art, file based drive pooling software.
Drive Bender - Drive Bender, class leading storage pooling technology
mhddfs - The driver combines a several mount points into the single one.
btrfs - Btrfs is a modern copy on write (CoW) filesystem for Linux aimed at implementing advanced features...
Greyhole - Greyhole is an application that uses Samba to create a storage pool of all your available hard drives, and allows you to create redundant copies of the files you store, in order to prevent data loss when part of your hardware fails.
EON ZFS Storage - EON ZFS Network Attached Storage