A few pianists still perform the Busoni arrangements of Bach's organ works - a corpus that includes about a dozen of the chorale preludes and three of the most popular free works (the inevitable D- toccata and fugue, the "St. Anne" prelude and fugue, and the C+ Toccata, Adagio, and Fugue). I don't perform the piano solo repertoire anymore, but I enjoy playing these over from time to time - if you know the organ originals, the audacity of Busoni's arrangements is jaw-dropping. Reading through the scores, you can hardly turn a page without encountering an unpleasant surprise - a sudden fortissimo corresponding to nothing in the original score, or a few measures of added arpeggiation, just to make sure you get the point. Purists would undoubtedly scoff at this, but Busoni's arrangements shouldn't be taken as a diplomatic transcription of the "original" - rather, they're reimaginings which are valuable primarily as a window into Busoni's aesthetic, and the performing practice of some 100 years ago.
Few people would lump Bela Bartok in with Busoni as part of the school of turn-of-the-century pianist-arrangers. Yet a quick visit to IMSLP turns up Bartok's piano arrangement of Bach's G+ trio sonata, as unpianistic a work as one could expect to find in the organ literature. What's more, the arrangement was published in 1930 - at which point Bartok was well into his middle age, and no longer performing regularly as a pianist. I don't know why Bartok made the arrangement, and would love to find out more about it (any Bartok experts in the room?), but in the meantime, I am fascinated to learn that the work exists, and delighted to have such ready access to it.