Neo Geo emulator turns the DS into an Advanced Entertainment System
Full sound emulation and (most importantly) multiplayer have yet to be implemented, but according to DCEmu, "Games like WindJammers, Metal Slug and Fatal Fury work either perfect or near perfect." Soon you'll be able to compare Metal Slug 7 to its predecessors on the same screen! If you own them, that is. Don't be a pirate.
[Via DCEmu; thanks to everyone who sent this in!]