I just walked past this tree, planted in someone's yard in southern New England:
The fruits are about the size of a hen's egg, and the leaves are long and broad -- maybe 8 in. (20cm) in length. Anyone know what it might be? I don't think I've ever seen anything quite like it.
That's a butternut tree (Juglans cinerea), a close relative of walnut trees. This is probably a hybrid variety such as Juglans × bixbyi. 100% pure butternut trees are quite rare, because they are highly susceptible to a disease called butternut canker. Planted butternut trees are usually canker-resistant hybrids between the butternut and another species of walnut.
The green, fruity looking part is the husk. Inside the soft husk is the hard shell that you would expect on a nut. Once fully ripe they will fall off the tree, and the hulls will eventually turn brown and rot away, leaving the shells visible. Butternuts are quite good eating, and much easier to extract the nut meat from than the common eastern black walnut. If come back in the fall and see that the tree owner hasn't harvested the nuts for themselves, it might be worth asking if you can collect some nuts off of their lawn. They might even be happy for you to do so, since the nuts can make it difficult to mow a lawn.
