The BS prefix is applied to games released for the Bandai-Sattelaview system in Japan (the system was an add on to the SNES in a similar way to the 64DD and the Nintendo 64). Quite a few games from the Legend of Zelda series were released on it. These games can be found floating around ROM sites with the BS Zelda prefix, if you look hard enough that is. Unfortunately, none of them are playable on emulators in their unaltered form. Some have patches available which make them perfectly playable. Some still do not load on emulators. To save you the disappointment...
- BS The Legend of Zelda - this is sometimes referred to as the Legend of Zelda Remix. Either way, it is a remake of the original Zelda game for the NES system. It has a number of patches available, and when all are used it is perfectly playable. See here for more details.
- BS The Legend of Zelda: Kodai no Sekiban - this rom is so large that it is available in 4 different parts. It is currently unplayable, although I understand there is a thriving community out there trying to make it work with emulators. Apparently the translation of the subtitle is "Ancient Stone Tablet" and you collect a piece of stone tablet from each dungeon. When it is patched and playable I will update this node.
- BS Zelda no Densetsu - Kamigami no Triforce - this is apparently the same as The Legend of Zelda: a Link to the Past. It plays a little (with a very similar title screen and opening sequence to Zelda 3) but it is impossible to create a save file, and so is unplayable. You might as well just get the ROM for A link to the past - it's perfectly emulated.
Since all these games were developed by Nintendo, I would hazard a guess that they are all very good. BS The Legend of Zelda is great, and if the third game is A link to the past in a different form then it will be great as well. When the other game is emulated better I'll let you know about that. In the meantime, to those of you into emulation I would recommend finding all these ROMs if you can, as they are very rare. If you find a working download, don't hesitate. You won't regret it.