I could not find this as already hinted, so I thought that maybe someone does not know about it and/or finds it "useful." Open up a Terminal window. It doesn't matter which shell you prefer; I use tcsh. Type in emacs and press Enter. Once it's loaded, press and release Escape, and then type x. At the bottom of the screen, you'll see M-x; type tetris and press Enter.
You're now playing Tetris within emacs. Use the arrow keys and space bar (to drop blocks) to play. When you are done, a highscore will be generated at /tmp. You can then exit emacs by pressing Control-X followed by Control-C.
[robg adds: I modified the above instructions to be a bit more detailed than as originally submitted. Also, if your $TERM is set to VT100 (in Terminal Preferences), you'll get a black and white version with 0's instead of the colored graphics version shown here. To see the colors, set $TERM to ansi.]

