We've seen lots of interesting ways to use old Macs, including the great Macquarium that pops up every now and again. But I think an ant farm is a new one -- that's what this 17" G5 iMac has been turned into. An eBay seller in Hawaii says he emptied out the Mac's insides and built a custom habitat to slide in and out of the unit, and then he filled it up with a special gel designed to grow and support a colony of ants. The only issue? He can't get the ants he'd need for the farm in Hawaii, so he's selling the whole kit and caboodle, ants not included.
The habitat is also watertight (though it's a little small), but if you wanted to use this as a Macquarium, that would work, too. Perhaps for Sea Monkeys? I think an ant habitat would be a little more original, however. Just hope that they don't get free. As of this writing, the price is set at only $1 with no reserve, so anyone who wants to try using an old repurposed iMac can jump at the chance.
AppleiMac 27-inch with Retina 5K display (late 2014)