Hot-swap drive bays are another option, particularly if you're using SATA. Note that there are versions that use drive carriers, and others that just allow you to slide in the bare drive. There are even 5-into-3 racks (5x3.5" drives in three 5.25" drive bays) available.
With the 3-into-2, 4-into-3, and 5-into-3 racks, you have to watch out for case compatibility. Some of the racks will have grooves in the side so that they can clear the tabs found in many modern cases, while others (like the Supermicro CSE-M35T-1) won't go into a case with tabs unless you bend them out of the way or grind them off. If you're using a case that uses old-school drive rails, it'll slide right in.
Supermicro also offers a SATA drive rack (CSE-M14) that fits four 2.5" drives into a single 5.25" bay.
One thing about the Supermicro racks - they're rock-solid, server-grade hardware, but they have LOUD fans more suited for a server room than a living room. If you use one, it's worth replacing the stock fan with a SilenX or other quiet fan.