These are tentatively referred to as "squirrel lemons" by many people although they are not lemons and definitely are not grown for the benefit of squirrels.
This is a fresh one. It's plausibly a type of chestnut.
All around it you can see the remains of another one on my steps. I made the mistake of wiping them off with my hand, which dyed my palm this disgusting shade of greenish yellow. The step is still a bit stained.
I found this broken squirrel lemon branch while on a walk. It probably fell during one of the storms we had recently. You can get a good look at the type of leaves this tree has.
This is a closer look. These squirrel lemons end up being used as projectiles by squirrels who rip them off their branches and drop them onto the unsuspecting passerby's below. The trees themselves can grow quite tall and the bombardment can hurt quite a bit should it hit you.