In addition to just musical instruments, a huge number of different hardware still use midi. I'm personally interested in DJ-world where things like DJ- and light-controllers use midi and therefore are very easily remappable and very hackable (if the sw is built correctly).
I've wanted to build some DJ-related sw myself that would work via browser but the fact that currently only Chrome supports the standard has so far kept me off it. I'd very much like to see Firefox supporting Web midi.