A few weeks ago when placing an order for something else I noticed this 'little' thingy. It is a drive bay for up to four 3,5" units that fits into the space of three 5,25" drive slots. It comes with a 120mm fan and has large rubbery washers between the centerpiece metal drive housing and the supporting side panels, that in turn are screwed or otherwise fitted to the 5,25" case mountings. The Cooler Master 4-in-3 drive module is originally made for their own CM-Stacker case series, but after the easy removal of the hard black plastic sliders it fits any normal case that can spare the space of those three 5,25" drive slots that it takes up. I put min in a typical Chieftec tower where it slid in easily on the plastic Chieftec mounting rails, leaving one spare slot for my DVD&CD-recorder unit.
More and better pictures can be seen in the Overclockers.com review of the CM-Stacker case:
http://www.overclockers.com/articles1087/index02.asp
Apart from the primary function to make it easy to fit up to four more harddrives in a case there are the extra features of vibration reduction by the rubber washers, and the fan for serious cooling if necessary. The rubber washers does their thing really well, because there is no need for having a screw stick to the metal of the inner drive bay. The washers rest on the metal holders that stick out from the side panels, without any metal touching metal. The reduced vibrations compared to a case with the typical metal-to-metal mountings is quite noticeable. Used with hard drives that has little idle-noise like Samsungs and Seagates, it can be a nice ergonomics upgrade too.
When it comes to the cooling, the 120mm fan it is not necessary to use when mounting up to two drives because there is so much air around the drives anyway. But with three or four drives that are being put to good use then it definitely comes in handy. It's not exactly quiet at normal speed, wich is about 1200rpm according to the manufacturer, but that could be regulated with the addition of a fan controller or other voltage adjustment. The fan is included in the package anyway and 120mm fans aren't exactly dirt cheap so perhaps you can use it for something else if you don't need it. The cost of the whole thing was just about $35, so it's definitely a thing to order 'on the side' of something else to warrant the freight cost. Personally after having said hello to this thing I do dream of getting an all-5,25" unit case the next time. The two 3,5" drive bays in my Chieftech case are basically just dead weight from now on.