Backblaze is a cloud backup service that needs cheap storage. Lots of it. They say a petabyte worth of raw drives runs under $100,000, but buying that much storage in products from major vendors easily costs over $1,000,000. So they built their own.
The result is a 4U rack-mounted Linux-based server that contains 67 terabytes at a material cost of $7,867, the bulk of which goes to purchase the drives themselves.
And best of all, they open sourced their hardware: