C-u M-x shell will do it.
It will prompt for a name for the new shell, just hit return for the default (which will be something like *shell*<2>.
Also works with eshell.
Another trick, if you use eshell: just as M-x eshell takes you back to *eshell* (rather than starting a new eshell), if you use a numeric prefix argument it will take you to that eshell buffer. For instance, C-3M-xeshell will take you to *eshell*<3>. Sadly if you use shell (rather than eshell), this trick doesn’t seem to work (in my Emacs 24.0.50.1 at least.)