The Konami Code is perhaps the most famous cheat code ever, a controller sequence that unlocked extra lives and virtual goodies in consolegames including Contra, Gradius, and Gyruss.
Save states have been a boon to 3DS owners not accustomed to the challenges of classic Nintendo games, and is a welcome addition to the full-sized VirtualConsole setup -- after all, how else are we supposed to get a decent score in Balloon Trip mode?