In past iterations of Animal Crossing, quality custom designs were a hot commodity. You could use them for shirts, hats, flags, wallpaper, and more, but making them well required meticulously coloring each pixel. Furthermore, sharing patterns required that you connect with the friend and give them the pattern. Thankfully, New Leaf now allows you to share patterns by QR code. Even better, by reverse engineering the code, there's already been an iOS app made that can convert images to Animal Crossing designs and can generate a corresponding QR code so that all you have to do is scan it.
The app is free, so check it out: QR MyDesign Assistant for Animal Crossing 3DS by Bochen Technology co. Ltd.
If you know of any other programs for PC or online that do the same thing, let me know! And feel free to leave any QR codes you've generated using this app in the comments. Here are the ones that I've made.
Update: An online app for making designs that's in Japanese. (Doesn't do QR codes.) (Link)
Update: Another one similar to the Japanese web-based app above, but in English. (Link)
Update: An Android version. (Link)
Update: An English downloadable version for PC is now available and in development. (Download (v.0.4 7/9/13), On Reddit)
Update: Another web-based app, but this one can generate QR codes. (Link)
Update: A web-based app that was submitted in the comments. Thanks, Jason W.! (Link)