The Incredible Spill Not from Edmund Scientific is a hanging saucer with a loop handle that prevents the application of horizontal force — thanks to the flexible handle — on beverage containers being carried with it. This is the force that typically causes liquid to slosh around and spill. The Incredible Spill Not is currently available for purchase via Amazon.
With the Spill Not, the liquid in the cups does not spill or slosh in the cup, and the cup doesn’t slide when carried or swung around. This is because the loop handle is very flexible and cannot deliver a lateral acceleration to the cup, which is usually what makes beverages spill from an open container not tipped.