Echinocactus grusonii

Echinocactus grusonii is a common 'golden barrel' cactus, easily available in stores. It blooms at a size of basketball or larger, and requires lots of sun. Here in Central California it can be planted outside, and grows well. It tolerates light frost, but when exposed to it the plants start developing unnatural side shoots.
Flower looks unusual for both Echinocactus and Ferocactus, with needle-like pointy petals. The blooms come from the felt that this plant develops at it's top once mature.
There is a lot of story around this species. Despite been probably the most commonly sold cactus, it's main habitat area is destroyed - sunk under the artificial lake. There are some newly discovered locations of this plant though. It is commonly accepted now that this species is actually of natural hybrid origin, with parents coming from Ferocactus and Echinocactus genera. The plant is commonly viewed as a golden ball, but really old plants in habitat are actually growing taller than 6 ft, with stem wrinkled like an elephant leg.