I have often used process substitution in zsh (and bash):

$ diff <(ls dir1) <(ls dir2)

This will create named pipes with the output of the commands and substitutes <(...) with the name of the pipe:

$ file <(echo lol)
/dev/fd/11: fifo (named pipe)

A neat trick I picked up in man zshexpn is that it’s also possible to have command substitution return an ordinary file by using =(...):

$ file =(echo lol)
/tmp/zshHQJRHb: ASCII text

To quote man zshexpn:

If =(…) is used instead of <(…), then the file passed as an argument will be the name of a temporary file containing the output of the list process. This may be used instead of the < form for a program that expects to lseek (see lseek(2)) on the input file.

This is nice to know when using programs that need to seek in their input files, e.g. unzip:

$ unzip =(curl -s http://foo.org/foo.zip)