We had to cut down an old Plum tree in the back yard about a year ago because it was dying. Apparently the fungus that lived in it is still flourishing. After a few wet days this week, it exploded in mushrooms.
Casual internet research (thanks, Danny) identified them as Honey Mushrooms (Armillaria mellea). They’re not dangerous (unless you’re a tree or cabbage, apparently). One site even listed recipies, but I’m not nearly brave enough for that.