How can I send the stdout of one process to multiple processes using (preferably unnamed) pipes in Unix (or Windows)?

^{Editor’s note:
– >(…) is a process substitution that is a nonstandard shell feature of some POSIX-compatible shells: bash, ksh, zsh.
– This answer accidentally sends the output process substitution’s output through the pipeline too: echo 123 | tee >(tr 1 a) | tr 1 b.
– Output from the process substitutions will be unpredictably interleaved, and, except in zsh, the pipeline may terminate before the commands inside >(…) do.}

In unix (or on a mac), use the tee command:

$ echo 123 | tee >(tr 1 a) >(tr 1 b) >/dev/null
b23
a23

Usually you would use tee to redirect output to multiple files, but using >(…) you can
redirect to another process. So, in general,

$ proc1 | tee >(proc2) ... >(procN-1) >(procN) >/dev/null

will do what you want.

Under windows, I don’t think the built-in shell has an equivalent. Microsoft’s Windows PowerShell has a tee command though.