For those who have not unlocked all the specialties . . .
MAGES
ARCANE WARRIOR
Source: Learned from a spirit in the ruined temple of the Brecilian Forest. Break phylactery in broken alter room (werewolf ruins, next to headless statue).
Bonuses: +1 Dexterity, +5 attack
BLOOD MAGE
Source: Taught by the Desire demon possessing Connor (talk instead of fight)
Bonuses: +2 Constitution, +2 spellpower
SHAPESHIFTER (unlocked)
Source: Can either be learned from a manual bought from Varathorn (Brecilian Forest Dalish Elves), or taught by Morrigan if her approval rating is Neutral.
Bonuses: +2 Constitution, +1 Armour
SPIRIT HEALER
Source: Learned from a tome that can be bought either in The Wonders of
Thedas (Market, Denerim) or outside Warden's Keep (Downloaded Content). Contrary to what you'd expect, Wynne cannot teach it.
Bonuses: +2 Magic, bonus to combat health regen
ROGUES
ASSASSIN
Source: Can either be learned from a manual bought from Alarith (Alienage, Denerim), or taught by Zevran once his approval is high enough
Bonuses: +2 Dexterity, +2.5% melee critical hit chance
BARD (unlocked)
Source: Can either be learned from a manual bought from Alimar, or taught by
Leliana once her approval is high enough
Bonuses: +2 Willpower, +1 Cunning
DUELIST
Source: Taught by Isabela at The Pearl, Denerim (hi dex, Zev or Lel present, "mess around")
Bonuses: +2 Dexterity, +1 damage
RANGER (unlocked)
Source: Learned from a manual you can purchase from Bodahn Feddic
Bonuses: +1 Constitution, +5% Nature resistance
WARRIORS
BERSERKER (unlocked)
Source: Can be learned either from a manual bought from Gorim (Market, Denerim), or taught by Oghren once his approval is high enough
Bonuses: +2 Strength, +10 hit points
CHAMPION (unlocked)
Source: Reward from Arl Eamon
Bonuses: +2 Willpower, +1 Cunning
REAVER
Source: Taught by Kolgrim if you defile Andraste's ashes (lose Leliana and Alastair in process, and maybe Sten)
Bonuses: +1 constitution, +5 stamina
TEMPLAR (unlocked?)
Source: Can either be acquired through a manual from Bodahn Feddic or taught
by Alistair once his approval is high enough
Bonuses: +2 Magic, +3 mental resistance