Thought this was pretty interesting:

Calc
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The GAME() function

Putting =GAME() into any cell results the text "say what?" in that cell.
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Frogger

Putting =GAME("Froggie") into any cell results the text "Froggie" in that cell.
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Tic-Tac-Toe

Put the =GAME(A2:C4;"TicTacToe") formula into A1 and press ENTER. A version of the Tic-Tac-Toe game is playable.
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Space Invaders

Put =GAME("StarWars") into any cell and press ENTER. A version of Space Invaders is playable. Although if you try to play it again, the message "oh no, not again" appears. To play again, you must fully close OpenOffice.org.


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Life, Universe and Everything

Put =ANTWORT("Das Leben, das Universum und der ganze Rest") into any cell and press ENTER. The result is the famous number 42 from the Hitchhiker's Guide to the Galaxy.

StarCalc team

Put =STARCALCTEAM() into any cell and press ENTER. The names and picture of the StarCalc developer team appears.

Random smileys

Put =TTT() into any cell and press ENTER. A random smiley appears. E.g. :-] ;-] B-) 8-}