Easily the most powerful energy conglomerate in the world prior to the Great War, Poseidon Energy was an umbrella corporation with its fingers in virtually all known forms of renewable and non-renewable power sources, as well as R & D for new, more powerful weapons, through their subsidiaries, so long as they remained profitable.
Though most of the public knew that Poseidon was a huge corporation and never particularly trusted it, very few people were aware that several companies owned by Poseidon were given lucrative defense contracts by the U.S. government – or rather, by Enclave members working as lobbyists to gain those contracts. Most of Poseidon’s most promising research projects never reached their final stages.
Though technically not part of the Enclave, many powerful corporations benefited from the Enclave's actions and their research facilities were protected during the atomic firestorm of 2077. Poseidon Energy was chief among these, as its facilities continue to be used by the broken remnants of the Enclave following the Chosen One's assassination of the Enclave President and the destruction of their Oil Rig headquarters off of the coast of California in 2242.
Many of Poseidon’s most promising projects never reached their final stages. In their paranoid dreams of a coup against the real U.S. government, the Enclave kept their special "toys" out of the American military's knowledge until it was too late to actually use them. For example HELIOS One where the tower could turn into an energy death weapon to wipe out anything in the U.S. at the time.
After the Great War, Poseidon no longer existed as a real force in the post-atomic world. However, technology scavengers like the Brotherhood of Steel and the Enclave still tried in vain to decipher Poseidon's plans and either confiscate this technology or turn them into working models, respectively.
Poseidon Energy was not a single, monolithic company, but was instead a cartel of various Poseidon-owned corporations controlling various aspects of the global energy market; the PoseidoNet data transfer system provided instantaneous communication between various Poseidon facilities including the Poseidon Oil Reactor № 5, the Enclave Oil Rig, possibly HELIOS One, and other locations throughout the United States and possibly the world..