ShreevatsaR is right, the answer is kill-emacs or save-buffers-kill-emacs, both of which are interactive, and so can be run from within Emacs with M-x save-buffers-kill-emacs. This is probably the best way to do it, since you will get to save modified files.
Another alternative is to make a shell file like this:
#!/bin/bash
emacsclient -e "(kill-emacs)"
Which you can run from wherever you like (menu icon, panel, etc).