He was famous for his visions of the future, of which many came to be. He was one of the first to work out geosynchronous orbits for communication satellites, dreamed of such (then) improbable ideas as what is now called the Internet and cell phones, and predicted a man on the moon by 1970. He's also famous for his "Three Laws:"
1. When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
2. The only way of discovering the limits of the possible is to venture a little way past them into the impossible.
3. Any sufficiently advanced technology is indistinguishable from magic.
He posted a farewell message to fans and family earlier this year on Youtube.