This game is pretty cool... definitely thought-provoking. You basically control this spaceman and you use your past actions to help you in the current time, by making use of a time travel machine. Just like in Back to the Future, the characters could see themselves perform actions of the past, you do the same in this game. But, be careful of paradoxes (I think they start becoming a problem around Level 6 or so).
Anyway, check it out: Chronotron
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OK, stuck on Level 16 for now. Time to take a break.