[Windows 10 Mobile] GoodDayToDie:
Great guide, [MENTION=2854207]snickler[/MENTION]!

A few things I've found useful:

Don't use the "Sirepuser" SSH/SFTP user for shell and file system access; it breaks deploying stuff to the phone (until you restore its default configuration). Instead, create a new user, give it the same username as your PC username, and set that one up (this is easy to do using the Interop Tools app). That lets you remote in to run arbitrary stuff, without screwing up your phone. It also means you don't have to supply a username anymore (at least for command-line tools); the client will use your username by default.

If you want to add a bunch of programs but don't want to worry about putting them in the MainOS partition (which is limited size and doesn't get cleared on hard reset), consider

C:\Data\Test\Bin

(or U:\Test\Bin, same thing), which is in the Data partition but also included in PATH by default.

Two programs to make sure you have in your phone's PATH (or otherwise easily reached) are TLIST.EXE (task list, similar to the usual "tasklist.exe" on Windows except inexplicably superior; for example, it supports giving a tree output showing process ancestry) and KILL.EXE (again, much like "taskkill.exe" on Windows except better). These are included in the Win10IoT System32 directory.

While SSH/SFTP have the huge advantage of security, the server on the phone kiiinda sucks. For much better performance, reliability, and compatibility, use TELNETD and FTPD, attached to this post. Copy the files to the phone, preferably somewhere easy like U:\Test\Bin. To start them, SSH to the phone, then run the commands

start TELNETD cmd.exe

start FTPD

Note that this will leave them running at all times; use tlist.exe and kill.exe to shut them down when finished or you give anybody on the same WiFi as you total control of your phone! You can use the telnet client built into Windows (you'll need to enable it from OptionalFeatures.exe), or use the one that comes with the Linux subsystem for Windows (bash.exe), or use a third-party one like PuTTY. For FTP, you *can* just use command line or browser-based (or third-party) FTP, but the best option is just to use the built-in support in Explorer; just navigate to "ftp://<YOUR_PHONE_IP>" and you'll see the root of the phone's filesystem. Advantages:

Both FTP and Telnet are really fast. SSH/SFTP is good, these are better.

No need for third-party clients, setting protocol levels, etc.

FTPD doesn't choke on large files the way the phone's SFTP server does, at least for me.

TELNETD supports things like "backspace" and "arrow keys" and other highly-advanced concepts. Tab completion works!

The obvious downside: there is no user authentication, no server authentication, no encryption, and no integrity checks. In other words, it is completely insecure. DO NOT USE THIS ON A SHARED WIFI NETWORK!

For Powershell, use TELNETD. Either run powershell after connecting to telnet, or just launch TELNETD pointing at powershell.exe. If you want to run multiple instances of TELNETD, you have to put them on different ports; add a second parameter with the port number.

start telentd C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe 2323

Tell your telnet client to connect on that port ("telnet <PHONE_IP> 2323"). For me, everything I've tried so far is working. No doubled input, I can see all the output, I can run programs, error messages are colored red, etc.