Here's Biederman's Gameboy to NES adaptor. I would have loved to have had this had it been released. Well, Famitsu did a little story on it.
And then they also did a little coverage of other interesting adaptors for the Famicom. The one on the left is, apparently, the 3rd version of the "Wide Boy" system that developers and magazines used to display GB games on a TV screen. The other two are adaptors to play FC games, and FDS games on the NES.
Here's Ninja Jajamaru looking slightly different from the released version.
Famitsu did a report on American games, and they covered the NES version of Sim City. Even though it looks pretty complete, an 8-bit version of Sim City was never released. This report seems to note that it would have been a US-only release anyway.