Nebulus on my A500.
Actually, my dad played it first. He picked the A500 up from Colchester Computer Center, near where I used to live, and took it back to the police station where he worked. He and his work mates set it up and played Nebulus, and they were absolutely amazed by the rotating "3D" tower. When we opened the A500 on Christmas day, that was the first game he said to play.
I still have very fond memories of that day, and still play Nebulus to this day :-)
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moto