Take, for example, this remake of Metroid II: Return of Samus for the original Game Boy. It may be nothing new for a fan to recreate an oldschool game, but this is one of the best we've seen. DoctorM64 not only did a great job with the sprites, but also tweaked the level design a bit so that the game flows more smoothly.
If you want to try this remake for yourself, it's only a click away. Keeping in mind that this is only a tech demo and not the completed version, we're sure that you'll be even more impressed. Also, you can check out DoctorM64's blog to get a feel for the creation process.