Can anyone help me out with this Super Fami cart?

So I picked this up today for a couple bucks. there is a cart like slot on the top. anyone knows what goes here?

 

It's a Satellaview cartridge.

http://en.wikipedia.org/wiki/Satellaview

8M memory packs are inserted into the cartridge slot on top to store downloaded data.

 

Yes:
http://superfamicom.org/blog/2010/04/internals-of-a-bs-x-cartridge/

 

Are BS carts with the likes of BS Zelda valuable then?

 

BS carts, empty aren't that expensive, but with games, especially BS Zelda they are. Mostly because they're no longer downloadable from the service for obvious reasons.