Also a form of CompactFlash device (originally built by IBM but now made by Toshiba and a few others as well) that amazingly fits an actual hard drive (complete with spinning cylinders, moving heads, and the electronic guts to handle CompactFlash interfacing) onto a standard CompactFlash Type 2 module that most CF Type 2-compatible devices can handle.
Originally the capacity was roughly 320MB, but new models are available with 1GB and 2GB capacities. 5GB capacity units are rumoured to be in existence in the lab and due for production soon.
These use more power than an equivalently-sized CompactFlash flash memory module (probably on the order of 25-50% more power consumption than a memory module), but the space these offer can be enormous -- imagine a digital camera that can take over 5,000 pictures before it runs out of "memory". That's what my Canon PowerShot S3 thinks it can get out of the 1GB microdrive card I've put in it. I've never even come close to filling it, even with 2,600 pictures on the card.