Citrahold is a 3DS Homebrew App, a piece of software, and a web service which allows you to easily sync your saves between 3DSes and Emulators.
Get started, check out the website!
For developers curious about the other repos which make this work (they don't get nearly as much attraction so I thought I'd link them):
I stopped actively working on Citrahold after January, but a lot more people have started using it so I may begin working on it again. It is still stable though!
I'm still yet to promote it anywhere, so not sure how you're all finding it but it's very cool!