nxmtp
Or memloader to do it from rcm mode.
sys-ftpd is good too for wifi transfer.
^ This guy knows what he's talking about.
If you're using TegraRcmGUI, you can load memloader directly from the tools tab. At this point your Switch is a glorified microSD card reader. It's probably the fastest way of transferring files to and from the Switch without ejecting the microSD card.
That being said, I don't really use it too much anymore. Sys-ftpd works great for transferring homebrew-related files wirelessly (I like using FileZilla as a client, but even Windows Explorer seems to work alright with it), and you don't even need to reboot your Switch to use it.
If you're installing NSPs, I recommend doing it over USB via Goldleaf. Very fast, it's much quicker than spending the time to copy the NSP to the microSD, and then installing it from there.