Addonics aims to make swapping hard drives as easy as changing floppy disks with its Snap-in SATA Mobile Rack. Mounted in a 5.25" drive bay, it lets you hot-swap any 3.5-inch SATA hard drive. The rack itself is $26.95, buy a bunch of drives and you'll be on your way to having unlimited storage.
Sure, we've seen hot-swappable devices before, but this is a good way to get the speed of SATA in a hot-swappable form factor for a low price. It's as easy as USB with no pesky rebooting required.