An astounding video made its rounds in January. A group of very experienced divers were on a dive off the coast of the big island of Hawaii. They were looking for manta rays when a wild bottlenose dolphin approached one of the divers. The dolphin was obviously in distress and the diver quickly saw that the problem was that a fishing line and hook were entangling the dolphin. The dolphin stayed still for the diver to begin cutting away the line, went up to breathe, and then came back down to the same diver so he could finish his work. He wasn't able to get the hook out of the dolphin's skin, but at least he was able to cut away most of the fishing line.
This begs the questions of exactly how could a wild dolphin know that a human could take care of whatever it was that entangling it, and how did that dolphin know that it could safely approach a human? One might also wonder how that dolphin - since it had of course never known what clothing is, much less diving gear - may have made the leap of understanding to know that the human-shaped figures it may have seen swimming near the shore or standing on boats are the same animal as the ones it approached who were wearing the scuba gear.
If this isn't hard proof of intelligence, I don't know what is. Now one might say that dogs approach humans when they're in distress, but this almost always refers to tame dogs, or dogs that have at least had a significant amount of exposure to humans and have often been bred for their loyalty and friendliness to humans. Dolphins, on the other hand, inhabit a wholly different realm than that of us landlubbers. What's more, we now know that dolphins - wild bottlenose dolphins like in the video above, at least - call each other by name. There's even research that's made some real progress towards not just understanding dolphins, but to engage in actual two-way conversations with them. This might well be possible since we've found that dolphin speech shares some similarities with human speech.