Tourists visiting the Kruger National Park in South Africa posted a video on YouTube of a crocodile trying to drag a wildebeest into the water. They struggle for a while but just as the wildebeest is about to go down, two hippos swim up and scare the croc away. Hippos are very territorial so that's probably why they did it, but it's still pretty amazing.
(The hippos come onscreen at 1:55 and make their move at 2:20. Warning: It might be hard to watch if you're an animal lover.)