Does the Library or anywhere on campus lend out pianos or instruments?

The short answer is no, not really.

However in the Digital Media Lab, one of the sound booths does have a keyboard in it.  This can't be taken out of the room.

There are pianos on campus that students may use.  There used to be one in the Student Union ballroom.  I know there is also one near the Roots Cafe in the Honors Dorm complex.  They are also pianos students may be able to use in the rehearsal rooms in the Music Department.

Off campus students could check out Gerry's Music Shop that rents instruments, this however will cost money and is located in South Hadley. There are other music stores in Northampton that might rent instruments as well.