Will.i.am's Puls smart band is a standalone wearable that lets you send and receive calls and messages, as well as post to Facebook and tweet, while on the go, but its bulky design and short battery life are turn-offs.
I'm pretty sure that the Samsung Galaxy Gear S does NOT work without a phone. In order for the Galaxy Gear S to work, it has to be paired with your phone. The phone doesn't need to be with you, but it has be powered on and connected to a cellular network somewhere. For example, your phone is at home but you are away at work with your Galaxy Gear S. If the two devices are paired and connected to the cellular network, then all is well. So, in a sense, it does work alone as a phone but in an important sense for me, it doesn't (You have to have or buy a phone in order for the Galaxy Gear S to function as a phone).