Fly Agaric, I think it's called. Classed as poisonous, and can cause hallucinations.
If you pick it, chop it, and leave it in a bowl of water overnight, you'll find most of the flies in your house will be drowned in the bowl in the morning.
I don't think they're particularly rare, but you don't see them that often.
They are pretty amazing to look at, though! I've seen them with 'scrape' marks across the top: apparently, squirrels sometimes have a go on them. Crazy squirrels.
I had something similar in a cup of tea once. Me and my mates were watching The Never Ending Story. That bloody flying dog kept staring at me, and jumping out of the TV.