Atavisms are perhaps one of the most visually compelling evidences for evolution, this is because they are generally very visual in nature. Examples of atavisms are human tails, teeth in chickens, hind legs in whales, dolphins and snakes, and extra toes in horses.
An atavism is essentially a trait that had disappeared in a species being expressed in an individual.
Here is a picture of a recent case of a human tail atavism in India.
As you can see, it isn't just a stump of a tail, but rather a fully formed tail, with vertebrae and everything. I assure you the picture is not a fake. There have been over 100 cases reported in medical literature over the years, some with vertebrae, some without.
Atavisms are strong evidence for evolution because they show us that the DNA for forming traits which have evolved themselves out of the population are still buried in the genetic code of modern animals. They show our evolutionary history, much like vestigial organs do. Creationist attempts to discredit atavisms, especially tails have undoubtedly failed miserably. Some creationists have claimed that human tails are just abnormal mutations, not realising that would be an argument for evolution, not for creationism. Some have also argued that the tails are not inherited, and that would therefore mean it wouldn't evolve. Either intentionally deceptive, or just plain ignorant, these claims are wrong. There have been cases where several generations have all developed tails, not only that, the genetic information to develop the tail has been inherited by every single generation in human lineage since we 'lost' our tails several million years ago. So the creationist arguments fail on every level imaginable, but that is to be expected from those deceitful crooks.
Here, you can find more information about human tail atavisms.
Here, you can find more pictures of human tails.