Coprinus comatus - Shaggy Inkcap
The Shaggy Ink-Cap! Extremely common, extremely interesting, and extremely tasty.
These, along with the Trooping Funnel, are one of the two fungi I have felt comfortable eating, simply because of how unique they are in appearance.
Find another tall, shaggy, white fungus that dissolves into ink - I bet you can't!
If you do forage them, get them young because they vanish into black goop remarkably fast!