Sonic the Hedgehog on Commodore 64 [video]
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This, and the Super Mario Bros port done a while back are excellent. The C64 is a particularly great 8 bit platform. Was capable of far more than intended, and it's always fun to see what people get it to do these days. I am surprised almost every year!
Back in the day, the REU (sp?) was a very interesting, but very underutilized and poorly adopted peripherial. I knew a couple people using GEOS seriously and they got something out of the device, but few others did. Now we see it's ability to do DMA exploited to great effect!
Who knew?