There’s an interesting item in the beta that looks like it may be an easter egg of sorts — the Black Mushroom. It drops from a clickable group of Black Mushrooms found within the Cathedral in Act I. The Black Mushrooms used to be found outside of Adria’s hut, but were recently moved to the Cathedral in a patch update.
Update – 05/18/12: The Black Mushroom is a reagent used in the recipe for reaching Diablo 3’s “land of rainbows & fluffy clouds, filled with hostile pink pony unicorns, teddy bears & flowers”, also known as…“Whimsyshire”. Read this guide to see what the Black Mushroom is used for.
I’ve read that the Black Mushrooms spawn in various places within the Cathedral, but I’ve only ever encountered them on Level 1.

When you click on the patch of Black Mushrooms, out pops a lone little Black Mushroom which you can pick up.

So what is the Black Mushroom..?
It’s probably nothing, but you never know… It could function similarly to Wirt’s Leg from Diablo 2, in that it may serve as a component used in unlocking a secret level in Diablo 3.
For right now, though, it’s mostly in the beta for a little nostalgia and comic relief. The Black Mushroom was a reagent from a quest in Diablo I, given by Adria the Witch. If you examine the tooltip, you’ll notice that it cannot be sold, and that Cain knows as much about it as he does red herrings.

My guess is that it’s simply just that…a red herring. It’s a kooky little item that Blizz stuck in the game as an easter egg – of which I bet there are many more yet to be discovered. It probably serves no other purpose than to cause players a moment of, “what the heck is this, and what is it doing here..?”
However…as I’d mentioned…you never know. Perhaps it’s more than that. Maybe the Black Mushroom will be a reagent for some sort of witches brew that is used to transport our heroes to a secret unicorn level – filled with pink demons and pretty rainbows.
One can only hope. ๐
lol – and let the unicorns be summoned o.O
Put it in a pot with Leoric’s Shinbone and make some soup?
If you’re not sure of the recipe … get an ingredient list from Izual, add some rainbow water and ring Wirt’s Bell to let the blacksmith know when dinner is ready. Then go for a hike toward Old Tristram with your new walking staff.
Is there a way to get rid of them? I have four just sitting in my stash taking up space… I can’t salvage them either… I know they are used to make a staff… But I don’t know how to get the items for that either… the only item I remember i need to create the staff is a bone from Cain’s knee(I think it was his knee…)… any and all help is welcome.
You can drop the item and then start a new game. It will disappear into the ether, freeing up your inventory slots. ๐ If you want to know what’s needed to craft the Staff of Herding, along with where to get the stuff, have a look at this post.