The Legend of Zelda: Ancient Stone Tablets is one of those forgotten spinoff games rescued from obscurity by a few dedicated fans. The game, thanks to BS Zelda, has now been translated into English (and French and German) and made available via emulator. Get it while it's hot.
The game itself is a remix of A Link to the Past, adding new challenges to the classic Zelda title.
It was previously only available via the Satellaview add-on for the Super Famicon (Japan's version of the Super Nintendo). The add-on allowed players to download a catalogue of old games (and some new ones, like Ancient Stone Tablets) via satellite connection that broadcast at certain times of day.
If you didn't catch it during the broadcast, you couldn't play. BS Zelda no Densetsu Inishie no Sekiban (or Ancient Stone Tablets) was broadcast over four weeks, with players saving each week's progress to cassette. I can see why even die-hards might have missed it.