The big blueberry, a Warlock's most lovable pet. Obtained at Level 10, the big blue is usually a Warlock's favored solo leveling companion because it can tank, having a hefty amount of health and the taunting abilities Torment and Suffering. Out of combat, it can regenerate lost health quickly with Consume Shadows, and in a pinch can be Sacrificed to grant a Warlock's version of Power Word: Shield. It isn't specialized for any tree, although a Demonologist will get the most bang for the blueberry buck.
The reliable Voidwalker won't be changing much in Wrath of the Lich King, but will be getting a few improvements with some talents from the Demonology tree such as the consolidated Fel Intellect and Fel Stamina, now called Fel Vitality, which rightfully increases Stamina as well as Intellect. Along with all demons, it also gains Avoidance, a baseline ability that helps it mitigate AoE damage by a massive 80%. Improved Voidwalker has not been changed, but a key talent that used to be perfect with the big blueberry -- Soul Link -- was reduced to a 15% damage absorption and no damage bonuses. The good news, however, is that Soul Link is now a Tier 3 talent, allowing almost any Warlock to pick it up for more survivability.