I believe we've only gotten 3 so far:
Bloomfang: -30% dmg for 2s, +1500HP
Qonker's Cube: +10% crit rate for 7s, +1500MP
Kindled Spirit: +10% damage & heals for 9s, Attack Speed +35
All have only been in a box once, no longer obtainable unless you find someone that still has one to sell.
Powerstone pets hover over your characters shoulder and can't be targeted and killed.