Archomentals are a storied race of beings from Dungeons & Dragons. They are essentially the "exemplars" (gods/leaders) of the Elementals.
Gary Gygax created the first archomentals - or, as he called them, "Princes of Elemental Evil" - in this edition. They basically served the same purpose as demon lords and arch devils, but for the elemental planes of Earth (Ogremoch), Fire (Imix), Air (Yan-C-Bin) and Water (Olhydra).
Planescape brought the archomentals back and gave them counterparts (since this was the Grid-Filling Edition), the Princes of Elemental Good; Sunnis of Earth, Chan of Air, Ben-Hadar of Water and Zaaman Rul of Fire.
The Archomentals were given their official name of "Archomental" in Dragon Magazine for this era, with the Archomentals of Evil appearing in Dragon #347 and their Good counterparts appearing in Dragon #353. This edition also introduced the concept of Archomentals of the Paraelemental planes, though only five are mentioned (Bwimb of Ooze, Ehkahk of Smoke, Chilimba of Magma and Cryonax of Ice) and only Cryonax got stats.
In 4e, archomentals, renamed "Primordials", got a HUGE promotion. With Points of Light mythos styled after ancient Greco-Roman myths, the multiverse was now created originally by the Primordials/Archomentals, with the Gods coming along later and adding permanency and mortal life to everything. This ticked off the Primordials, and so they fought the Dawn War. The gods killed and imprisoned a huge bunch of Primordials, and others were corrupted into the first Demon Lords, and so the Dawn War came to an end.
Because 4e went for the Elemental Chaos rather than the rigid grid-system of Planescape and its Elemental, Quasimental and Paraelemental Planes, its Primordials were a very diverse bunch, with figures like Atropus (imagine a frigging planet made of undead flesh 'n' bone and you got the right idea), Iktha-Lau (embodiment of deep space and the empty void) and Maul-tar (a freaking huge-ass dragon-thing of stormclouds and lightning chained down by rune-etched chains with links the size of castles).
For a tentative list - and do note that some of these appeared in earlier editions as either Archomentals or elemental gods...
- Mual-tar the Thunder Serpent (“Lords of Chaos: Mual-tar,” Dragon 370)
- Erek-Hus the King of Terror: Erek-Hus cleaved the dragon-god Io into two equal halves with his adamantine axe, but was then defeated when the two halves became Bahamut and Tiamat. Source: “Dragon”, Monster Manual.
- Bryakus: Defeated by the gods long ago, the spilled blood of Bryakus gave rise to the first hydras. Source: “Hydra”, Monster Manual.
- Vezzuvu the Burning Mountain: She is trapped in a field of radiance that keeps her shifted partially out of reality, unable to affect or be affected by her surroundings. Source: Manual of the Planes.
- Balcoth the Groaning King: His head was cut off by the gods, but it continues to survive as ash and cinders in a cave somewhere in the Elemental Chaos. Source: Manual of the Planes.
- Fire Lord Imix: Primordial served by fire titans. He also has a temple of beaten gold called the Eternal Flame Pavilion in the City of Brass, and goblin cults in the world. Source: Manual of the Planes.
- Shangar the Uncrowned: Primordial served by fire titans. Source: Manual of the Planes.
- Solkara the Crushing Wave: This primordial is trapped in a glacier. Imprisoned by Melora, her exarch Sekolah, Kord, and Sehanine. Source: Manual of the Planes and The Plane Below.
- Castanamir the Shattered Khan: Primordial served by storm titans. Source: Manual of the Planes.
- Olhydra the Rain Queen: Primordial served by storm titans. Source: Manual of the Planes.
- Haemnathuun the Blood Lord: His corpse, a 1,000’ long mass of red stone, floats in the Astral Sea. The Blood Lord created the abominations known as blood fiends in his own image. The Keening Delve, near the City of Brass, was once his stronghold. Haemnathuun was slain by Bahamut, Bane, and Ioun. Source: Manual of the Planes.
- Heur-Ket: A blustery primordial of wind, thunder, lightning. Source: “Hestavar: the Bright City,” Dragon 371.
- Tabrach-Ti the Queen of Bronze: Tabrach-Ti was slain by Achra (later called Bane), the first primordial to fall in the Dawn War. Source: “Deities and Demigods: Bane”, Dragon 372.
- Piranoth the World Mover: Primordial of earth, fire and frost. Source: Revenge of the Giants.
- Tmesus the Black Star: Primordial of dissolution. Source: E3 Prince of Undeath.
- Zurtharak the Vein of Iron: A primordial who could burrow through solid stone, Zurtharak was defeated by Moradin (with the help of Kord and Bahamut) wielding the “God Hammer” Guldarak. Source: Hammer of Vengeance (avenger paragon path), Player’s Handbook 2.
- Nekal of the Glowing Deep: A primordial. Source: Concordant Leader (warlord paragon path), Martial Power.
- Ilmeth: A primordial who regretted the lives he took and was slain by his own followers in the Dawn War. Source: Vestige of Ilmeth (warlock power), Arcane Power.
- Shax the Devastator: Dead primordial of the sea. Source: Vestige of Shax (warlock power), Arcane Power.
- Atropus the World Born Dead: Possibly an undead primordial. Source: Open Grave.
- Syvexrae the Queen of Stone: A primordial whose petrified but still-regenerating body serves as Doresain’s palace in the White Kingdom. Source: Open Grave.
- Queen of Chaos: A primordial. Source: Dungeon Master’s Guide.
- Nihil: Primordial of unbeing, defeated by Bahamut. Source: The Plane Above.
- Gargash: Primordial of the Underdark, mortal enemy of (and defeated by) Torog. Source: Underdark.
- Iktha-Lau the Ever Empty: A primordial of utter cold from the darkest voids of existence, said to still roam free. Source: The Plane Below.
- Ulctilantilokla: A primordial that only partially exists in reality, birthed from the multiverse’s first dream, also roams free. Source: The Plane Below.
- Tziphal the Mountain Builder: Primordial of folding earth, petrification, and volcanoes, is said to have created many of the creatures that possess the power of petrification. He was defeated in the Dawn War and imprisoned in a barrow in the Elemental Chaos, shifted slightly into the past so the barrow cannot be tampered with. Source: The Plane Below.
There's also this big list of Primordials that appeared in "The Plane Below" sourcebook for 4e, which WoTC generously put up on their website for ease of access: http://www.wizards.com/dnd/files/excerpts/excerpt_element1.pdf