Short-beaked Common Dolphin
Delphinus delphis

Short-beaked Common Dolphins are found across much of the world, such as Europe, the southern and eastern Pacific, Japan and the north-east coast of the US and Canada. They can occur in huge pods (even into the thousands - we saw a pod of 1,000+ off Sydney) and together, groups will hunt fish and squid. Thy can be quite acrobatic, jumping bow-waves of ships and flipping out of the waves. They are closely related to the Long-beaked Common Dolphin, which is found usually only in smaller regions of the tropics. Identifying these dolphins is fairly easy when good views can be had due to the unique pattern on the flanks.
 

1: Sydney Pelagic, Tasman Sea, NSW, 14/08/2010.
2-3: Sydney Pelagic, Tasman Sea, NSW, 12/03/2011.