Here's a list of exarchs for the PoL setting. Let me know if I've missed any; corrections and updates are welcome.
Exarchs of the Gods
Asmodeus:
?
Avandra:
Aasterinian (Draconomicon)
Bahamut:
Kuyutha (Draconomicon 2)
Vivexkepesk (Draconomicon 2)
Xerefri (silver dragon exarch) (Dungeon 174)
Bane:
Hruggek, the Master of Ambush (MM)
Maglubiyet, the Battle Lord (MotP, Dragon 372, MM2, Plane Above)
Thurkearvaeri the Nightdancer (Draconomicon 2)
Corellon:
Astilabor (Draconomicon)
Erathis:
Edarmirrik (Draconomicon 2)
Gruumsh:
Baghtru, Fist of Gruumsh (demigod exarch) (MotP, Plane Above)
Luthic, the Blood Moon Witch (MotP, Plane Above)
Vaprak the Rapacious, the King Eater (MotP, Plane Above)
Yurtrus (MM2)
Ioun:
Athearsauriv, Eye of Heaven (Draconomicon 2)
Zuoken (Dragon 378)
Kord:
?
Lolth:
Eclavdra (MM3)
Lolestra (Revenge of the Giants)
Melora:
Sekolah (MM, Plane Below)
Moradin:
Berronar Truesilver (MotP, Divine Power)
Clangeddin Silverbeard (MotP, Divine Power)
Dugmaren (Divine Power)
Dumathoin (Divine Power)
Gorm Gulthyn (Divine Power)
Haela Brightaxe (Divine Power)
Marthammor Duin (Divine Power)
Sharindlar (Divine Power)
Torugar (Revenge of the Giants)
Vergadain (Divine Power)
Pelor:
Hainard (leader of Pelor's White Guard) (MM2)
The Golden General (unconfirmed) (AV2)
Sehanine:
?
Tharizdun:
?
The Raven Queen:
Achreisis (Dungeon 155)
Chronepsis (Draconomicon)
Tiamat:
Chillreaver (white dragon exarch) (Dungeon 165)
Ephelomon (former red dragon exarch) (Dragon 377, Dungeon 168)
Kurtulmak (former servant, demigod) (Dragon 394)
Mornujhar (black dragon exarch) (Dungeon 170 & 171)
Namissi (blue dragon exarch) (Dungeon 174)
Nefermandias, the Forsaken Pharaoh (former brown dragon exarch?) (Draconomicon)
Quanathrik, (blue dragon exarch) (Dungeon 175)
Urum-Shar (fallen demigod) (Draconomicon)
Virizan (naga exarch?) (Dungeon 166)
Vraxanault (red dragon exarch) (Dungeon 168)
Vulkur Vaal, Vaal the Flayer (demigod, independent?) (MotP)
Torog:
Duon-eel the Thrice-Slain (missing, imprisoned?) (Underdark)
Jerran (undergoing transformation to full exarch) (Underdark)
The Abnegator (primordial exarch) (Underdark)
The Cringe (Underdark)
Vecna:
Falazure, the Night Dragon (Draconomicon)
Kas the Betrayer (former exarch) (Open Grave)
Vocar the Disobedient (former exarch) (MotP, DMG2, E2)
Zehir:
Atlathessk, the Seneschal of Night (MotP, Plane Above)
Others:
Erevan Ilesere (member of the Seldarine) (MotP, Plane Above)
Fenmarel Mestarine, the Lone Wolf (member of the Seldarine) (Plane Above)
Lunia (dead, former exarch of Sehanine?) (AV2)
Ometh (former exarch of Nerull?) (Open Grave)
Shevarash the Black Bow (member of the Seldarine) (Plane Above)
Exarchs of Demon Lords
Baphomet:
Asterion, the Thief of the Maze (Dragon 369)
The Misbegotten (Dragon 369)
Codricuhn:
The Voice of the Storm (Dungeon 172)
Dagon:
?
Demogorgon:
Kazuul (former Champion of Erathis) (MM2, Plane Below)
Thrarak (MM2, Plane Below)
Orcus:
Doresain, the Ghoul King (MM, MotP)
Elder Arantham (self-proclaimed exarch) (Dragon 364, E1)
Holchwier (dead) (Dragon 364)
The Lord of Zombies (dead?) (P2)
Thrullzon (dead) (P2)
Turaglas:
Gargmanethka (Dragon 376)
Yeenoghu:
Nezrebe, the White Knight (Dragon 364)
Exarchs of Archfey
The Prince of Frost:
Mournwind, Sister of Lament (Dragon 374)
Soulsorrow, Sister of Lament (Dragon 374)