Sea Anemone and Clownfish
This symbiotic relationship is famous and is often studied in high schools, because it was made obvious in the movie Finding Nemo. This relationship is known as mutualism, where both species gains benefit from having the other one with it. The protects the clownfish hides within the poisonous arms of the anemone, and also leaves it some morsels of food leftover from its own meals, which the clownfish eats. To earn its keep, the clownfish removes parasites from the sea anemone, scares away predators, and provides nutrients to the anemone via its excrement.
Whale and Barnacle
This would be the other more famous relationship, this one known as commensalism. The whale gains absolutely nothing from barnacle attaching to its body, yet the barnacle is also harmless. It is the same as humans hitch-hiking. The barnacle benefits greatly for it is a filter-feeder, and doesn’t need to spend any energy bringing the food to its stomach.
Three more examples are over at AquaViews, the SCUBA magazine