Terminal boasts some cool, but hidden features that we like to call "Easter eggs." These "eggs" are hidden features that the developers build in (mostly for fun or laughs), that don't have anything to do with the functionality of the program.
One of these "eggs" is the ability to play Tetris in Terminal. Here's how to do it. First, open a new Terminal.app (/Applications/Utilities) window and type "emacs," then hit enter. After the screen loads, press "escape," then "x." Type "tetris" (lower-case) and hit enter. There you have it, a fully playable game of Tetris. Use the arrow keys to control the blocks.
Want more tips and tricks like this? Visit TUAW's Mac 101 and Terminal Tips sections.