The Drobo from Data Robotics is one of the first in a new wave of user-friendly protected storage options. It is essentially a smart external set of drive bays that give you the functionality of seeing one logical disk from your computer as well as protection from a single hard drive failure, a la RAID.
The Drobo is a sleek black "robot" that contains 4 drive bays capable of taking 4 SATA hard drives of any size. It connects to a PC or Mac (Linux support is spotty, but you can get it to work) via USB 2 and is very easy to install and set up. No extra software is needed, it is detected just like a flash drive, although Data Robotics provides a software "dashboard" for extra functionality.
Because it contains data protection, keep in mind that you will essentially lose the largest drive in terms of capacity for the purpose of protection. That is, if you have a 750 GB, 500 GB, and 500 GB drive in it, the total available storage will be 1000 GB (this is a general rule of RAID due to striping and maximum theoretical data protection). The main benefit of Drobo is that it will allow you to add future drives with no configuration if you need to upgrade, and if a drive fails, you can just take it out and put a new one in. Your data is always accessible whether a drive fails, you're replacing one, or adding new storage.
Similar products like the ReadyNAS, which is a bit more expensive, have benefits of working over a network, but the Drobo can be attached via storage link to a router such as the Airport Extreme for network access. The Drobo is fairly new and has a relatively small user base, but help on the forums (called drobospace) is plentiful.
One disadvantage to the Drobo is that it slows down as it gets close to full, because it's constantly trying to optimize where data is and such, a problem that more conventional systems don't have. Data Robotics is apparently working on a NAS (networked) version of the Drobo, but no details are known.
For someone who wants a hands-off protected storage solution, the Drobo is perfect, but it's definitely for a power user. An average computer user who just wants some additional storage/protection should probably use a single external enclosure or even DVD backups.