How can I get a list of all open named pipes in Windows?

In the Windows Powershell console, type

[System.IO.Directory]::GetFiles("\\.\\pipe\\")

If your OS version is greater than Windows 7, you can also type

get-childitem \\.\pipe\

This returns a list of objects. If you want the name only:

(get-childitem \\.\pipe\).FullName

In Powershell 7, the second example \\.\pipe\ does not work (see this known bug), so you will have to stick to the first syntax.