It’s a SATA drive “toaster”.
Heard about these on MacBreak Weekly. It’s a dock for internal hard drives. Buy your SATA drives on the cheap, keep them in their original packaging, and when you need to use one, just pop it into this dock.
Keep them cataloged with an app like CDFinder. And store them on a shelf — remember, one copy at your home or office, and one copy offsite! — out of the way until you need ’em.
Most consumers are not going to need these, but let’s say you shoot a lot of video. This is a nice alternative to multiple Drobos, or more expensive external hard drives (of uncertain reliability).
Erick just found this Blacx SE dock, which has a cover to protect the docked drive, and a powered USB 2.0 hub to boot.