The 3DS's Virtual Console adds an extra layer of abstraction today, with the Game Boy port of the NES version of Adventure Island II (called, confusingly enough, "Adventure Island.") You can enjoy this Game Boy version of an NES game in a little Game Boy-looking window on your 3DS!
DSiWare features Double Bloob, a vertical shooter/puzzle game we first covered way back in 2007, years before there even was a DSiWare service. It's gone through a lot of changes since then, not the least of which was a departure of the development team from its former home at the infamous Nibris.