NES Classic Edition, you'll be happy to know it can now be jailbroken thanks to console hackers in Japan and Russia allowing you to soft-mod it and upload Nintendo Entertainment System games via USB cable with a guide below!
To quote from ArsTechnica.com: According to the whiz kids at Reddit's NESClassicMods community, the solution won't work until you've created a save file in Super Mario Bros' first slot. (Chances are, you've already done this just by playing the game, since creating game saves is so easy with this system.)
Once you've done that, connect your NES Classic Edition to a computer via a micro-USB cable, then boot the NES in "FEL" mode. This is done by holding down the system's reset button while pushing down the power button from a powered-off state. While you're booting, you should also run a "sunxi-FEL" interface on your computer. (An open-source version of compatible "USBBoot" software can be found here.)
The rest of the steps (via Pastebin) land firmly in "operate at your own risk" territory, as they require copying your NES Classic's internal data to your computer, then modifying and adding files via an application made by hackers. Doing so, by the way, includes the dubious step of supplying your own ROM files, which you may have either dumped from your own cartridges or downloaded from other Internet users.
One tool linked from that Reddit community, however, comes with two open-source NES ROMs that are in the legal free-and-clear to upload to your hardware.
Code:1. Go here http://linux-sunxi.org/FEL/USBBoot#Using_sunxi-fel_on_Windows and install driver with zadig. Fel mode is activated by holding down reset while switching on power button - http://linux-sunxi.org/FEL 2. Grab tool here https://github.com/madmonkey1907/hakchi/releases Dump uboot and kernel from your nes. uboot should have md5 b13b538f6b7c86623ceea69c456d4eb5, if it's not, it might be unsafe to continue. 8ce1857653874e8f0cd3fe68abe1bb4a and 9b8046b47c947d9fa322976c038003e0 also confirmed to work. 3. Prepare roms - with nesmini shell, rom is a directory, not a file. Template+tool here https://www.dropbox.com/s/3t3wrkhw26shro0/nesromtool.zip?dl=0 tl;dr you need a rom, image, and optional *.desktop file. it's a text file you can edit. 4. Place rom dirs in mod/hakchi/transfer/rootfs/usr/share/games/nes/kachikachi/ nesromtool will do this for you if you unpack it in nesromtool subdir of hakci-gui tree 5. unpack kernel / repack kernel 6. Do a memboot, nes will shutdown, switch it to fel mode again. 7. Flash kernel <- do this ONLY ONCE. there is no need to ever do it again. until custom kernels appear that is. If there is a save in first slot of Super Mario Bros. it will load mod, otherwise it will load stock shell. You also get getty on uart interface, but there isn't much use in it if you don't have uart soldered. DISCLAIMER: I'm not responsible for any damage you might inflict upon your nesmini
Once you've added your own game files, which should also include custom JPGs that will appear in the NES Classic's "box art" GUI, you'll have to repack the hardware's kernel, then fully flash the hardware yourself. (Again, we remind you, these kinds of technical steps can result in a bricked NES Classic if anything unexpected happens.)
Do all of those steps correctly, and you'll see every single game you've added appear in the slick, default interface.
Thanks to @ArthurBishop for sharing the news in the PSXHAX Shoutbox!