Most Macintosh hardware products do a good job of hiding the actual electronic parts inside sleekly designed cases, and for most people, that's probably best. But if you want to go beyond the basics, to soup up a Mac past the stock configuration, or troubleshoot certain problems, sometimes you need to get down to bare metal.
Take hard drives. Cases provide physical protection, a certain level of useful industrial design, and conversion from the hard drive mechanism's native power and interface connectors to standard power jacks and ports such as FireWire and USB 2.0. But making it possible to power a hard drive mechanism and connect it to a computer doesn't require a case, just the connector conversions. Several new products now enable you to do just that - use a hard drive mechanism directly on a Mac or PC without a case.
Why would you want to? Perhaps a friend or client is switching from a dying PC to a Mac and needs to transfer content from the PC's hard disk to a VMware Fusion or Parallels Desktop installation on a new iMac. Maybe, working as a consultant or help desk support technician, you regularly encounter situations where you need to make backups of or recover data on disks installed in computers that are otherwise non-functional. Or perhaps, like me, you've had trouble with a FireWire drive case or drive bay used for regular backups. The problem isn't with the drive mechanisms, but you're still dead in the water unless you can get those drives to mount.
I haven't needed to use these products extensively, but I and a friend have successfully used, which costs $29.95, and the slightly more expensive , which runs $39.95. The two devices seem essentially identical, in that they provide flexible power and connectivity cables for all 2.5", 3.5", and 5.25" drive mechanisms, whether they're IDE or the newer SATA (I or II). You're unlikely to find many 5.25" hard drive mechanisms these days, but the products work with bare CD and DVD mechanisms as well.
To my mind, the main downside of these devices is that they're USB-only, which makes them significantly less useful with older Macs that rely on FireWire for high-speed connections and have only slow USB 1.1 ports. USB 1.1 runs at 12 Mbps, USB 2.0 at 480 Mbps, and FireWire 400 at 400 Mbps. Even though USB 2.0 isn't as fast as FireWire 400 in real world usage, it's plenty sprightly for drive use, whereas USB 1.1 is painfully slow.
WiebeTech sells a pair of products that address this lack of FireWire compatibility, but they come at a price and with limitations. Their and  products are boxes that attach to the back of a 3.5" IDE bare drive, providing power and connectivity, and then connect to a host Mac via FireWire 400 (for the FireWire DriveDock) or 800 (for the ComboDock). They also include power switches and feedback LEDs. Unfortunately, WiebeTech's docks cost noticeably more at $99.95 for the FireWire DriveDock and $169.95 for the ComboDock. And if you want to use the ComboDock with drives other than 3.5" IDE mechanisms, you need  that cost between $49.95 and $99.95 (the full kit with all six adapters costs $499.95). Although I've not used the WiebeTech docks personally, my impression is that they're aimed more at the technician working with four or five different drives every day, rather than someone who needs to access a bare drive only occasionally.
But what about the other function of a hard drive case: physical protection? It's certainly true that you could install a bare drive in an inexpensive hard drive case, but most cases lack the interface flexibility of these bare drive adapters, and it's often fussy to insert and remove drives from cases. The WiebeTech docks come with a bottom plate to protect the drive electronics (and you can purchase additional plates if desired). But Granite Digital has a better answer to this problem:, available either in stretchy silicone ($9.95) for quick insertion and removal or aluminum ($19.95) that offers more protection and cooling for longer term use. A package of the silicone Drive Shields includes shields for both 2.5" and 3.5" drives; the aluminum Drive Shield works only with 3.5" drives.
The bottom line is that if you ever find yourself needing to work with bare hard drive mechanisms, one of these inexpensive adapters will prove an essential addition to your toolkit.