Wanna see something cool? Of course you do. Take a gander at THIS!
That's SNK vs. Capcom: Match of the Millennium for the Neo-Geo Pocket. That wouldn't be anything out of the ordinary, except it's not running on a Neo-Geo Pocket. Nope, this is on the Nintendo 3DS, courtesy of RetroArch.
RetroArch is a jack of all trades emulator, similar to MESS but with a more intuitive interface and some neat bonus features. On more powerful machines, you can use filters to make handheld games like this one look the way they did on the actual hardware. (I covered this in greater detail last May, for those curious.)
The Nintendo 3DS doesn't have the power or the resolution to pull this off, but just being able to play these games is a big step up for the system. Before RetroArch, a hacked 3DS could run games from a small pool of systems, and not always very well. Now, it's compatible with a dozen consoles, and the emulation is generally faster and more reliable.
Here's a snapshot of Snow Bros. for the Sega Genesis. M2 ported a handful of Genesis games to the 3DS, but I doubt they've got any plans for this one. You're not likely to see sleeper hits like Mega Turrican or Dragon's Fury (aka Devil Crush MD) as Sega 3D Classics either, but they run perfectly well with RetroArch. You can even play Sega CD games, in case you've got a desperate urge to make some color-deficient music videos with Kriss Kross or Marky Mark Wahlberg.
RetroArch also handles arcade games, including Capcom's Forgotten Worlds. I'm not entirely sure how you'd play this without a dial... your hero just seems to randomly turn on his own, without accommodations for the missing controller. Still, it looks gorgeous on that 3DS screen! I expect similarly dazzling results from CPS2 and Neo-Geo games, although I haven't tried the former and have had no luck getting the latter to run. (I'm sure it's possible; I just don't think I've got files in the right place.)
The WonderSwan got a brief mention elsewhere, so I guess I should point out that its games also run on RetroArch. I haven't found a single game in the system's library that has given me trouble, aside from titles like Judgement Silversword which force you to flip the system sideways. (Thanks for that, Gumpei. Yeesh.)
Game Boy Advance titles, on the other hand, are less reliable. Double Dragon Advance and Pac-Man Collection run nearly as quickly as they do on the actual system, but Castlevania: Circle of the Moon is stuck in slow motion, possibly because of the game's heavy reliance on sound samples.
It's also worth mentioning that I tested these emulators on a New 3DS. Your results will probably be less encouraging on older systems, not only because the hardware is less powerful but because what's there is less accessible to homebrew programmers.
Between all that and the fact that RetroArch can't be accessed directly from the home menu, the 3DS still has a lot of ground to cover before it can challenge the PSP for the title of best pocket-sized emulator. Still though, it's a big step in the right direction. Anyone who's already hacked their 3DS (preferably the New model) ought to give it a try.