What handheld gaming concoles do you guys know so far?

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Hand held gaming systems have been there since the 70's. As the time has passed by you know that these systems went on evolving. From monochrome gaming device, now you can have the game plays in the palm of your hands. There are a lot of hand held gaming devices which are there in the world. May be more than 100 types of them. So I want you guys to tell me the systems which are rare .