The first Mage specialization, the Arcane Warrior, is something of a Warrior/Mage hybrid. These powerful sorcerers are capable of wielding higher-level weapons and armor, thanks the the ability to use their Magic stat in place of Strength to satisfy usage requirements.
Specializing in Arcane Warrior grants an immediate +5 bonus to Attack and +1 to Dexterity.
The Arcane Warrior specialization is unlocked during the course of the Nature of the Beast questline. On the lower floor of the Brecilian Ruins, there is what appears to be a ruined library of some kind. There will be a broken stone altar in the room. In one of the corners you will find a glass phylactery. Upon touching it, your character will experience the memories of an ancient Elven warrior. He will offer to teach you the specialization in return for setting his spirit free. Simply place the phylactery upon the altar to do so.
The Blood Mage specialization is unlocked during the Arl of Redcliffe storyline. To receive it, the player must be a mage and offer to enter the fade to track down the desire demon that is responsible for the Arl's sickness. During the mission, choose to speak with the desire demon rather than killing her. She will offer you the Blood Mage specialization if you choose to make a deal with her. This is the only way to unlock it.
Blood Mages can use health to power their spells rather than mana, drain the health from their allies to increase their own, and can even control the minds of weaker-willed opponents. Blood Magic is illegal, and Wynne will leave the party if you reveal to her that you are a Blood Mage.
Blood Mages receive an instant bonus of +2 to Spellpower and +2 to Constitution.
Shapeshifters receive an instant bonus of +2 to Constitution and +1 to Armor. These crafty Mages are capable of altering their physical form, becoming a bear, spider, or swarm of insects at will. These abilities can be significantly improved by learning the Master Shapeshifter talent, as well.
Unlock the Shapeshifter talent by speaking to Morrigan before you've raised your approval rating with her to 30. Once you've raised your relationship above neutral, however, she will not teach it to you.
You can also buy the specialization manual from Varathorn in the Dalish Camp. The book will cost you about 12 gold.
Spirit Healers are by far the best healing class in the game. Mages can throw out a healing spell here and there, but the Spirit Healers can keep your party alive and fighting like no other.
Specializing in Spirit Healing grants an immediate bonus of +2 to Magic as well as a minor in-combat health regeneration bonus.
To unlock the Spirit Healer specialization, purchase the manual from the Wonders of Theadas shop in Denerim for around 15 gold. Wynne will also teach you the specialization, if you raise your approval rating with her high enough.
Page three has the full rundown on Warrior specializations in Dragon Age, including Reaver, Templar, Berserker, and Champion.