Just regular old desktop hard drives, there are some Barracuda's in there, a Spinpoint, and a couple of WD greens.
Some for file storage, some for auto backups.
At the time I didn't know the difference, and I bought whatever was cheapest. I would have upgraded to proper archive/enterprise drives if I felt that I needed them, but these have been fine *touch wood*.
I'm sure that the correct equipment would be best, but a hard drive won't just disintegrate if you don't use it for its intended purpose.