In the Action Camera space, the big name is really GoPro. But GoPro isn't always the best for action cameras. Their products are really good, and continue to get better, including the new GoPro Fusion, but there are plenty of other options available right now, for action cameras. Including YI, AUKEY, DBPOWER and many others. These all provide great products at cheaper prices than GoPro. Like the YI 4K+ Sports and Action Camera for example. It sports 4K at 60fps quality video, and it is almost half the price of the GoPro HERO5 Session which can also do 4k/60fps. YI also has a few other options which are in the list below, including the YI Lite which is under $100.
Other manufacturers (read: smaller manufacturers) have been able to lower the price and still include a slew of great accessories to go along with the action camera. From different mounts, to extra batteries and even modified selfie sticks (or as some might say, monopods). These make using these action cameras even better, as you are able to take them out to the pool, the beach or really anywhere and put them on anything or anyone without needing to buy other mounts and such to get started. Makes the entry price a lot better. For instance, if you wanted to mount a GoPro on a dog and see what a dog's day is like, you might need to buy a headstrap, but with something like the DBPOWER model listed below, it is included in the price.
Action cameras are quickly becoming really good alternatives to using your smartphone for getting pictures and video in different situations. Especially since a lot of smartphones still aren't waterproof, you'll see GoPro's at a lot of beaches and pools around the world. And these are some of the best action cameras that are currently available. If you search Amazon, you'll find a ton of others, but some of those are rather sketchy, while the ones here are not, and will actually provide you with a good experience using its action camera. Which is why you want to buy one anyways.