This is likely the most addicting game I have ever played.
The only problem is that it does not work on Vista.
It is free and harmless... to your computer at least.

http://www.thewayoftheninja.org/n.html
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All there is to thinking...
Is seeing something noticeable
Which makes you see something
You weren't noticing,
Whaich makes you see something
That isn't even visible.

PM me if you want to play N.