Over at Emacs Redux, Bozhidar Batsov has a short post on using Meta+! to run a shell command from the minibuffer. The output can go to various places—see Batsov’s post for the details—but usually what you want is to specify the prefix argument (Ctrl+u Meta+!) so that the output of the command is inserted at point.
It’s a useful command but I hardly ever use it. I’m much more likely to use shell-command-on-region (Meta+|) which pipes the contents of the buffer into a shell command. Analogously to Meta+!, specifying the universal argument will cause the contents of the buffer to replaced by the output of the shell command.
Emacs has so much builtin functionality that you probably won’t use these commands very often but sometimes they’re just what you need.