This must be called the Butterfly Agave because of the shape of the leaves, not because the blooms attract butterflies, since as you can see by this comma-infested sentence, and the picture below, there are bees swarming the blooms on this plant.
There are lots of Agaves in bloom in Berkeley these days (Like this one).
This Agave potatorum is only about 2 feet across and 10 years old, but this is the end for this beautiful specimen. The bloom stalk is about 12ft. tall. You can see how much energy it would take for this giant stalk to come out of this rosette:
And the flowers are kind of nice too. At least the bees think so.