What are some alternatives?
When comparing Greyhole and unRAID Server, you can also consider the following products
StableBit DrivePool - State of the art, file based drive pooling software.
OpenMediaVault - OpenMediaVault is the next generation network attached storage (NAS) solution based on Debian Linux.
Drive Bender - Drive Bender, class leading storage pooling technology
FreeNAS - FreeNAS is a storage operating system strong and robust enough to meet the needs of enterprise level businesses.
mhddfs - The driver combines a several mount points into the single one.
Rockstor - Rockstor is a free and open source NAS (Network Attached Storage) operating system.