This is rather sad. The Baiji, also known as the Chinese or Yangtze River Dolphin, is extinct. This is the first known species of cetacean (whales, dolphins and porpoises) to go extinct in recorded history, and its demise was directly caused by humans, and therefore completely avoidable.
The Baiji looked kind of like a small Bottlenose dolphin (think Flipper) with a longer snout. It lived in the Yangtze River system in China, which suffers from intense development pressure. I read somewhere that nearly fifteen percent of the world's population lives along the Yangtze. Dams and other changes to the natural river flow damaged the Baiji's habitat, overfishing reduced its food supplies, and poaching, pollution, and collisions with ships did the rest.
Losing any species is unfortunate, but losing a large and relatively intelligent mammal is particularly bad. There are three remaining species of river dolphins, and I hope that the extinction of the baiji will prompt more efforts to protect them.