How to pipe content in Emacs' buffer to external program, and print the result?

Use shell-command-on-region which is bound to M-|

e.g.:
M-| sort | uniq -c RET

With a prefix argument, the region is replaced by the output of the shell command (which effectively adds all shell commands to Emacs' editing toolkit; very useful when you know how to do something outside of Emacs, but don't know an equivalent native function).