They're simply called citrus juicers, and you can see them pictured above. You put the citrus half in the well of the juicer cut-side down (counter-intuitive, we know!). The dome part of the juicer flips the citrus inside out and squeezes every last drop of juice. Holes in the well let the juice out while trapping seeds and larger segments of fruit.
Our friend says that he gets way more juice using this tool than anything else he's tried. It also directs the flow of the juice really well so it goes into the shaker instead of dripping all over his hands or the counter. Plus, it's easy to use so our friend doesn't get tired when that order for a third round of cosmos comes in.
The juicers come in three sizes intended for oranges, lemons, and limes, though the lemon juicer works well for both lemons and limes. They're coated with enamel and are dishwasher safe. You can find them at many kitchen supply stores.
Find It! Citrus Juicers, $13-$16 at Sur La Table
Related: Five Good Uses for Your Citrus Peels
(Image: Sur La Table)