Samson makes extremely tiny wireless systems. You’re never going to see them on a Hollywood set, but for their target market, they’re nice little units. They make a waterproof earpiece for exercise instructors that I’ve used in the past with no problems.
Here they are demoing their Go Mic Mobile, a credit card sized receiver that plugs into smart phones, and receives 2 channels! It’s another of these 2.4GHz units, and it comes with a built-in rechargeable battery. Pretty impressive for the cost.