Greenpeace is a real-life organization that's occasionally shown throughout The Simpsons. It's a non-governmental, non-profit organization that "ensure the ability of the Earth to nurture life in all its diversity." They deal with environmental issues throughout the world including global warming and pollution. They're infamous for both protest and illegal actions against large corporations negatively affecting the environment.
Greenpeace has been referenced numerous times throughout the Simpsons. The first appearance was during Burns' Heir. Waylon Smithers, Jr. leaves a sponge upon Charles Montgomery Burns causing him to drown, inducing a flashback. Among other clips, one of Greenpeace is shown. Two green peace members are preparing to sink a ship carrying huge amounts of nuclear waste. As they get ready to do so, Mr. Burns sinks their boat by putting a hole into the boat. He then escapes by swimming away. He then wakes from his hallucination.
Greenpeace was also involved in a raid at Joan Bushwell's Chimpanzee reservation after they gained information about Bushwell's role in enslaving chimps for a diamond mine, although they briefly entered conflict with the Simpson family after she convinced them that they were poachers.