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minecraft!
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it's a very succesful little jewel, having just sold 1 million copies after reaching beta.
it's created by markus persson, aka notch using java.
it is NOT free. you can try the creative version for free, but i strongly recommend buying the beta. it costs 15 euros (if i remember correctly)
minecraft has no goal. get used to it. what you do is you appear in a randomly generated, beautiful world so the adventure is different each time. you then have all day to create a shelter, then at night, monsters will come out... (sounds a little like stranded, right?)
yes, except for one thing... the entire world is made out of various blocks: dirt, grass, stone, sand etc... you use specific tools to collect specific blocks. you can place them back down to create any type of structure. after a while, the survival aspect of the game disappears (because you live in a castle you made yourself) and instead you just keep building, mining, exploring and attending to your every whim.
it's really fun!
check the website out at: www.minecraft.net
or this awesome fan made trailer here: http://www.youtube.com/watch?v=v5fS4RF3ht0