Japanese Ice Ball mold. I kept reading about how they supposedly kept your drinks colder longer due to the reduced surface area, and how they were coveted by serious mixologists everywhere. Me, I was mostly enthused by the idea of a big frozen spheres floating in my drink.
We weren't in the market for the fancy $200 model that carves out the ice, and even the $16 dual mold at the MOMA store seems a little too pricey. I went for the $12 for 6 spheres option (plus shipping) on eBay, and I'm happy to report that these trays work beautifully.
They're easy to use and take up very little space in the freezer, and will probably replace regular cube trays for us. It takes about 90 minutes for the ball to melt completely in a warm room, so it keeps your drink nice and cool for awhile. Plus there's the terrific side benefit of it being awesomely pleasing to look at.
One tip: be sure to put the frozen ball into your glass and then pour your liquid on top to mitigate the exciting-but-traumatic cannonball effect. Don't ask me how I know.