Happened just this summer. I'm not sure I entirely "saved" their life, but at least eliminated the risk for it. I was camping with some friends and we decided to go up the waterfall (small 20-25-foot falls) for a few hours. The falls aren't too dangerous, but a 16-year-old had just drowned there some four or five weeks earlier, so I guess there's always some risk.
Most people cross the stream at the top of the falls to hike down to the bottom, and if you cross right at the top the riverbed is mostly large, smooth slippery rocks, and most people slip on their way across. After we were done jumping the falls, we were up at the top drinking beer and laughing at people slip and fall before telling them to walk upstream a bit before crossing. Then, a group of three girls, look to be about freshman or sophomores in college, come out and they are giggling and slipping all over the place. They keep going across, slipping and giggling the whole way, and just as they get past the slippery bit, one girl slips hard and starts getting pulled into the mouth of the waterfall. This triggers the other two to begin screaming bloody murder, which makes me jump up and check out what's happening. I see the girl starting to get pulled into the fall. Without a whole lot of thinking (I was pretty familiar with the falls), I jumped up and ran over to help. The mouth of the fall first takes a four foot drop, makes a short S-bend and then drops the 20 feet to the pool below. I jump completely over the first 4' drop, and brace myself behind her, pinning my feet into a rock at the bottom and grabbing a couple rocks on either side with my hands, so essentially she's being pinned against my body by the force of the water coming down the fall. Then, a couple of my friends are able to come over and help pull her up.
While there's no way to really know if I saved her life or not, I at least saved her from having a potentially even more traumatic experience than it was. She was very thankful afterwards.