Any mediocre nature documentary will have examples of mutualism - fish which live in coral and eat the predators of the coral etc.
The most interesting examples are when parties seem to act against their own selfish interests.
The classic example is a small fish which lives by picking pieces of meat out of a large fish's teeth without being eaten. Naively we might expect that the larger fish will always go for the immediate meal, and eat the small fish.
In fact it is in the long-term interests of the larger fish to do without one meal in order to have its teeth cleaned (and parasites removed).
Such cases are confirmation of cooperation between selfish entities for mutual gain.
Mutualism is not to be confused with symbiosis, which is a group depending on each other for their very survival.
See