This is probably a stupid question, but I'm not really a techno whiz and I don't really have that much knowledge about emulation or how it works but whatever. Why hasn't 64DD emulation happened yet? It doesn't seem like it would be that difficult. I've seen stuff like "64DD EMULATOR COMING SOON LOLOLOLOL" but nothing really solid. Or maybe it's happened and I'm being stupid, so forgive me for being completely dumb. Either way answers would be nice.
There aren't many great titles, thats the problem, and the hardware is rare. No one is ever motivated enough to finish it because it just isn't worth the time they spend on it.
There is an N64 emulator out there that recently added support for the N64DD but I haven't been able to get it to work. I forgot what it was called but I'm sure there are other on here who can point you in the right direction.
There's a post with the games and the emulators on this very forum. Look in the Nintendo Development section.
there is one mess has just started supporting n64dd game so has 64drive see http://bsxproj.superfamicom.org/64dd/dump/
Flashcarts are using cart conversions of the disk games. Some emulators like PJ64, Mupen, and Cen64 support running these. The only emulator that will run the actual disk images is MESS/MAME.