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You can download the software for your PC or Mac, or just run it off of the deviantART page since that didn't seem to work for me.
Forget lithographs or coffee mugs. You need to get this. It's a replica Portal Gun signed by the development team. All proceeds go to charity (Child's Play). And at time of posting, it's on eBay for a surprisingly reasonable price of $760.
Considering the worth of something like that to super-fans, and the place your money is ending up, that's a steal.