This is a 63K download of a deceptive number square game from the East called Sudoku
The computer sets random numbers up "1 to 9" in 9 squares of 9 squares you have to arrange the numbers in all rows(vertical & horizontal) that they have 1 to 9 (no duplicates) and all the 9 squares have "1 to 9" www.sudoku.com (example here)
This will get the brain cells working http://oomz.net/iSudoku/(free download)
They are starting to publish these in the papers
Here are more links to this game Sudoku and there will be other downloads in this search(if mine is not so good?)

this is a free online version
www.mousebreaker.com/games/sudoku/

I have been doing these daily and have only got 2 filled all week!(can take anything from 20 minutes to a few hours)
ps There is a highscore table on this site But there are names with "0" against them
A real thinkers game

This is a do it yourself version very playable (pass it on)
www.phon.ucl.ac.uk/home/mark/sudoku/workpad.html