Great topic.  Painters: Braque is a good example of someone who was in the shadow of Picasso, but was a wonderful artist in his own right, and who people only remember for his Cubist period.  Some come and go: Caillebotte keeps being rediscovered, and then isn't again.  Perhaps the greatest watercolourist ever was Richard Parkes=Bonnington, who only specialists know: he died young, but was massively influential (especially to Delacroix).  Everything he painted was magical.  Every once in a while someone notices Joan Mitchell, the great American abstractionist, and then she goes undiscovered again.  Most recently, in the MOMA Abstract Expressionist exhibition, her painting ("Ladybug") stood out, and was commented on in the press. Maybe this time.  
Writers: Ivan Bunin. Ivan Bunin. Ivan Bunin.
Musicians:  The Boswell Sisters,  as opposed to the Andrews Sisters (whom most people have heard of), but the Boswell sisters are far superior, and real jazz musicians;  the great blues singer, Son House, has been overshadowed by Robert Johnson (his student), but is in a passionate league of his own ("Death Letter Blues" is on Youtube.  Nothing more passionate, he just throws himself at his grief -- he was a drunk and in his seventies when this was recorded).
Other works:  Two movies:  "The Moon's Our Home" which no one has seen in many years (it is barely available on VHS) starring Margaret Sullavan and Henry Fonda.  A beautiful, screwball comedy which hardly ever shows up in the mammoth discussions of more famous films of the type.  Margaret Sullavan is forgotten except for "Little Shop Around the Corner", but she was a fine actress.  A second film, better known, but still obscure, is "Out of the Past" with Robert Mitchum and Jane Greer,  a classic film noir that many people miss.  The best femme fatale ever. 
Novels: hardly unknown to the French, but "Le Grand Meaulnes" is still obscure to North Americans.  Maybe because it is so French (Watteau in words).  But then lots of astonishing world literature is unknown to English speakers: Leopardi doesn't exist. Heine only exists in the song lyrics.


But sometimes keeping them obscure and a pleasure of your own is part of the fun.  I had an uncle long dead who was obsessed with Jan Breughel, the son of the more famous Breughel, and whenever he went to a museum, he would find the obscure room where he might find a couple of Jan Breughels (mostly flower paintings, but lots of other fine works as well).  I find that I now do the same, and have pleasure in finding a Jan Breughel or two in some corner.

Forgot to add that Robert Browning had a famous, very, very beautiful poem called "A Toccata of Galuppi's" in which he deals with this very topic: an obscure "old" music.  

Hi Edna - loved your Galuppi.  Thank you.  Artists pass in and out of fashion and in performing art, this is particularly so.  Impresarios are always on the lookout for the next artists they can propel into prominence.  Even Mozart, I believe, dropped out of fashion sometime after his death up until about the 1950s.  I believe the introduction of widely available recorded music had something to do with his revival and with keeping him in the public hear, so to speak.  Speaking of recorded music.  I had always lamented not being able to hear Paganini perform (d 1840) and thought that Rachmaninoff (d 1943) was similarly unrecorded, only to find, much to my delight, that he was quite extensively recorded, and I am now the proud possessor of one box set of his solo recordings. Much joy and amazement at the aplomb and skill and sheer musicality with which he deals with very complex pieces.  It's as if he just fits seamlessly into the music.  I have some quite old recordings of Segovia.  His effortless performances remind me of Rachmaninoff's performances.  In the bustle of supporting the current generation of uber technicians, performers such as these two, and I have no doubt others I could also name, become forgotten.  They should not be forgotten, particularly by the uber technicians, who could learn a thing or two from their artistry.     But what a pity about Paganini - just 100 years between not being recorded and being able to be recorded. Weep!