Euphorbia horrida
Native range: Southern Africa
Known names: African Milk Barrel
Hardiness: Keep above freezing
Mature Size: Grows slowly ~5' tall, as wide as you'll allow it to be.
Light: Direct sun, Half day sun- avoid hot afternoon sun. 
Water: Water during summer (let soil dry out completely), keep dry during winter.
Soil: Cactus/Succulent mix
Dormancy Period: Winter
Pet-Safe: No
Plant Size: Grown in 4" pot, shipped semi bare root
 
This may look like a cactus but it is not a true cactus, and is actually more related to milkweeds. Produces offsets at the base. Avoid getting sap on you, especially around your eyes.