Easily the most powerful and influential energy conglomerate in the United States 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 technologically advanced weapons, through their subsidiaries, so long as they remained consistently profitable.
Although most of the pre-War public knew that Poseidon was a huge corporation and never particularly trusted it, very few people were actually aware that several companies owned by Poseidon were given lucrative defense contracts by the U.S. government – or even, by the Enclave. Most of Poseidon’s most promising research projects never reached their final stages however.
Though ultimately not apart of the Enclave, many powerful corporations benefited from the Enclave's actions  and their research facilities were shielded from the atomic firestorm of 2077. Poseidon Energy was one of these, as its facilities continue to be used by the the Enclave following the Destruction of Control Station ENCLAVE.
After the Great War, Poseidon no longer existed as a real force in the post-atomic world. However, highly technological factions such as 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 rather 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, and other locations throughout the United States and possibly even the world itself...