There are a lot of composers, instruments, and musical ideas out there that are not as well-known as they ought to be, often for the silliest of reasons. Some are ripe for rediscovery, some were doomed from the outset.
For rediscovery, I'd like to nominate Thomas Davis, Baroque composer of flute sonatas, published in London; his sheet music is available on IMSLP, and looks pretty reasonable to me. But I have no idea who he was, and haven't been able to find any sign of his music being performed or recorded (though I'll admit I haven't tried terribly hard).
I suspect he was doomed because his name is too common; a search for that name in London in ancestry.com finds 8758883 people during the period 1841-1911 (obviously the wrong period altogether, but giving some idea of the sort of difficulty associated with finding a common name in a large city). If he'd only been called Giovanni Pietro del Castanetti and born in a small village outside Udine, his sonatas would be all over YouTube.
In the same vein, Johann Ernst Galliard deserves an honourable mention for having a name indistinguishable from his product (sharing his name with a dance).
For sheer unlikeliness in musical objects, I'd like to nominate Carlo Giorgi's keyless flute of 1888, with its 11 finger-holes. I'm counting...
Anyone else got a favourite person, instrument, or musical idea that should be remembered?