Thottbot has been outdated since Burning Crusade, I compiled this list from wowhead (appropriately
) and came up with the following 142 obtainable heads. Seeing as some are drops and/or quest starters it is possible to have more than 25 at a time, but the meat and bones of the list (so to speak) are quest-only drops, I'd be surprised if your collection could exceed 40 significantly. Still, that's two frostweave bags worth of heads
A Shrunken Head
A Talking Head
Aldaron's Head
Ambassador Malcin's Head
Apexis Guardian's Head
Archeologist's Shrunken Head
Artorius's Head
Athrikus Narassin's Head
Barak's Head
Baron Longshore's Head
Boss Grog'ak's Head
Caliph Scorpidsting's Head
Channeler's Head
Chief Sharptusk Thornmantle's Head
Chieftain Zul'Marosh's Head
Chok'sul's Head
Dar'Khan's Head
Darkreaver's Head
Defiant Orc Head
Epoch Hunter's Head
Falconcrest's Head
Fardel's Head
Felendren's Head
Fenwick's Head
Firebeard's Head
Gallywix's Head
Gan'zulah's Head
Garrick's Head
Glommus's Head
Glrggl's Head
Gobbler's Head
Gor'tesh's Lopped Off Head
Grimscale Murloc Head
Grom'thar's Head
Gurok's Earthen Head
Gurubashi Head Collection
Hairy Herring Head
Harvester's Head
Head of Alexi Barov
Head of Argelmach
Head of Arugal
Head of Balnazzar
Head of Bangalash
Head of Baron Rivendare
Head of Baron Vardus
Head of Bazil Thredd
Head of Cho'war
Head of Deepfury
Head of Forgefiend Razorsaw
Head of Gammothra
Head of Gath'Ilzogg
Head of Grimson
Head of High Cultist Zangus
Head of Jammal'an
Head of Kael'thas
Head of Kel'gash the Wicked
Head of Kelris
Head of Krom'zar
Head of Lathoric the Black
Head of Magus Rimtori
Head of Nagaz
Head of Nefarian
Head of Onyxia
Head of Ortor of Murkblood
Head of Ossirian the Unscarred
Head of Overseer Maltorius
Head of Ramstein the Gorger
Head of Rend Blackhand
Head of Targorr
Head of Tel'athion
Head of the Broodlord Lashlayer
Head of the Corrupter
Head of VanCleef
Head of Weldon Barov
Hetaera's Beaten Head
Hetaera's Bloodied Head
Hetaera's Bruised Head
Hex Shrunken Head
Hezrul's Head
Human Head of Ras Frostwhisper
Ingvar's Head
Ivar's Head
Kenata's Head
Khan Dez'hepah's Head
Khan Jehn's Head
Khan Shaka's Head
Klinfran's Head
Knucklerot's Head
Kurzen's Head
Lar'korwi's Head
Lescovar's Head
Luzran's Head
Magtheridon's Head
Marcel's Head
Marez's Head
Mechazod's Head
Melenas' Head
Mimiron's Head
Mirefin Head
Muckrake's Head
Mug'thol's Head
Murloc Head
Nazan's Head
Nek'rosh's Head
Nezzliok's Head
Ol' Sooty's Head
Omokk's Head
Ordanus' Head
Or'Kalar's Head
Otto's Head
Pamela's Doll's Head
Pickled Murloc Head
Prospector Anvilward's Head
Raptor Head
Ribbly's Head
Sack of Murloc Heads
Sarilus Foulborne's Head
Serena's Head
Severed Night Elf Head
Shadow Lord Fel'dan's Head
Shadumbra's Head
Shrunken Head
Simone's Head
Socrethar's Head
Solenor's Head
Stormwatcher's Head
Stuffed Shark Head
Targ's Head
Thaelis's Head
Tharil'zun's Head
The Head of the Hand of Kargath
The Head of the High General
The Hexxer's Head
Thule's Head
Valroth's Head
Vejrek's Head
Verog's Head
Vile Familiar Head
Worg Master's Head
Wrathtail Head
Zaeldarr's Head
Zalazane's Head