Typically what you'd see called viable PVE raiding specs in vanilla:
Warrior: Prot (Tanking) or Fury (DPS)
Druid: Resto (Healer)
Hunters: Marksman
Mages: Arcane/Frost until AQ40, then deep(?) fire is a good option. One or two guys with imp blizzard.
Paladin: Holy (Healer) with one or two guys with 21 in prot for Blessing of Sanctuary
Shaman: Resto (Healer)
Priests: Holy or Disc/Holy (Healer), one Shadow Priest (DPS)
Rogue: Combat or Combat Daggers.
Warlock: SM Ruin, DS Ruin, or MD Ruin.
But it's often the case that a dedicated player with a quirky build may very well do better than the Johnny-know-nothin' with the cookie cutter build. Couple of out-of-the-ordinary builds you sometimes see:
Hemo Rogues: Doesn't scale well IIRC? Something like that.
Feral Druids: Scaling and itemization problems, having a rogue with a brez/innervate is nice but the DPS isn't nearly as good. As tanks, they have truckloads of health, but can't parry or block, which is a problem.
Hybrid Druids: 30/21 feral/resto might be the one half-decent hybrid healer build I guess.
Arms Warriors: The "I don't want to respec for PVP" build. NO MORTAL STRIKES!
Smite Priests: Scales surprisingly well, but doesn't really bring anything to the table that isn't already covered by Mages or Warlocks.
Ret Paladins: Bad DPS and don't listen to the guy with 2000 max mana trying to convince you that he can make up for that with off-healing
Other Shamans: (???) I don't know the horde so well